Fire spits from the muzzle of a Russian-made machine gun. Assault rifles join the fray.
Leftist Revolutionary Armed Forces of Colombia (FARC) guerrillas and Colombian counterinsurgency troops trade shots across a gorge.
On a nearby plateau, 100 metres of thick brush separate two other rebel squads from their adversaries.
Grenades echo as they explode.
"It's tough fighting in all this mud," said a guerrilla named "Adrian", who flinched with every shot he or his comrades fired. " This is to slow the army's advance. Within two or three days they'll take up new positions and we'll fight them," he added.
The battleground that day was an insignificant hilltop in El Porvenir, a tiny hamlet in eastern Meta province. The firefight lasted close to an hour - another skirmish in a string of anonymous battles that, these days, rarely make the media headlines.
Another testimony, too, to the cat-and-mouse nature of this, Colombia's almost five-decade-old conflict.
On paper, government security forces currently appear to have the upper hand.
The lethal 2008 strike on Raul Reyes, a member of the FARC's seven-man leadership council, was a warning of the devastating military air campaign ahead.
Months later, the army's "Operation Checkmate" freed kidnapped former presidential candidate Ingrid Betancourt and 14 fellow hostages and was a severe blow to the FARC's domestic and international political ambitions.
Then, when the air force - backed by army and police commandos - killed the FARC's top field marshal Jorge Briceno, alias "Mono Jojoy", in a "pinpoint attack" in September 2010, the government began to trumpet the end of the road for the rebel force. Officials estimate FARC membership has dropped from more than 20,000 fighters a decade ago to fewer than 7,000 now. The guerrillas themselves have not published their own figures.
In April, the head of the Colombian armed forces, Admiral Edgar Cely, said: "The FARC is in its death throes - although this perverse organisation refuses to believe that and fights using terrorism, explosives and minefields."
Those military setbacks in the years following the collapse of peace talks in 2002 propelled the FARC back to the depths of the jungle, virtually beyond the reach of the media.
It was only after several weeks of driving an old Jeep through remote corners of eastern and southern Colombia, leaving notes in obscure farmhouses, that I was able to reestablish contact with the guerrillas.
When they responded it was with a rare invite to spend ten days marching alongside one of their so-called "mobile companies", a unit whose main function is to fight.
Despite the string of recent defeats inflicted by the government, none of the 54 young men and women combatants of this FARC unit, the "Marquetalia Company", were talking about surrender.
"Mono Jojoy dies and it's like everybody is dead. Comrade Manuel Marulanda (a veteran FARC leader) dies and again it's like everybody is dead. That's what they think but it's not true," said "Jagwin", commander of the newly reformed Marquetalia Company. At 35, he's the oldest in the unit. He said he was the son of peasant farmers and joined rebel ranks some 20 years ago.
"We're sad because Mono Jojoy was like a father to us. But it's like what happens at home if your father dies, there's always a brother who will replace him and run the farm," he added.
The conversation with "Jagwin" is cut short.
An Blackhawk helicopter gunship clatters overhead, searching for the rebel column that just attacked army troops in El Porvenir. As it whirls overhead it spits up to 4,000 rounds a minute into the jungle canopy from its six-barrel Gatling gun.
The rebels call this helicopter the "harpy", a reference to the violent winged spirits of Greek mythology.
The Marquetalia Company pulls back with just one walking wounded.
Colombia's war is not a war of positions. In hamlets like El Porvenir there is little to defend. Steep cattle pastures, a humble schoolhouse with broken-down desks and pockets of thick rainforest.
The retreat is laboured. After heavy tropical storms, mud is ankle deep. Fighters clamber up and down slippery hillsides carrying backpacks full of clothes, ammunition and food - weighing around 30kg.
Camp that night was a banana grove. Government aircraft constantly circled.
Guerrilla commanders ordered a total blackout and confiscated flashlights from fighters. All spoke in hushed voices.
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As they listened to engines droning overhead they would whisper "the explorer", reference to a reconnaissance plane, or "the pig", a Vietnam-era AC-47 gunship bristling with weaponry and night vision equipment.
Their lives depend on spotting those aircraft in time and avoiding detection.
Under the tin roof of an abandoned peasant shack, Jagwin explained how he survived an aerial bombardment.
It was past midnight back in August 2009. He and his comrades heard the whine of a fleet of fighter-bombers approach and then their camp exploded in flashes of light and a storm of shrapnel. He saw the silhouettes of fellow fighters and heard their screams as they tried to flee.
"Our only option was to dive into the trenches. When the bombardment started we practically buried ourselves in those holes and when they started to strafe with gunfire and troops began disembarking then we ran to escape," he said.
In that attack alone, he said, 33 of his fellow guerrillas were killed.
Willinton, deputy commander of the Marquetalia Company, has also felt the fury of air raids. He gave few details but confessed that he had no option but to leave dead and wounded behind – a taboo in any military force.
"It's tough to have to flee the battlefield or escape a bombardment and leave wounded or dead companions behind. They were my comrades. It's tough but it was an exceptional circumstance. Sometimes you just have to do what you can to escape," he said.
For that reason, this night and every night, commanders briefed combatants - and me - about evacuation routes in case of bombardment. They instructed us on using shallow riverbeds and small foxholes dug alongside their sleeping quarters to shelter from a potential shower of shrapnel.
The following two days were a series of gruelling marches.
These combatants were mostly in their twenties, from poor backgrounds and very fit.
But rainy season had set in in Meta province and they advanced at little more than two kilometers an hour.
Nobody was much in the mood for talking en route, weary under backpacks, assault rifles and mortar tubes.
Their rubber boots squelched, half-filled with brackish river water, half-filled with sweat.
Vast tree roots formed natural staircases down muddy banks. Electric blue butterflies flit between the trees. Howler monkeys swung overhead and occasionally lobbed down branches.
With limited access to TV and radio and marching for days under a thick jungle canopy, it's easy to lose track of time. Days become weeks and blend into years. The FARC's revolution has become a war with no apparent end.
Beyond the confines of the rainforest, these young men and women would have been part of the Twitter generation. Yet most, like 21-year-old Eliana, had joined rebel ranks before they were 15 and were way too poor to afford a computer.
Eliana said she joined the FARC when she was just 13. She ran away from home after a fight with her mother and roamed the streets of an eastern Colombian town "getting into trouble", as she described - without further elaboration.
She's slim built and admitted she could "hardly carry the groceries home from the store" before becoming a guerrilla. Now she boasts she can march for days with a 30kg backpack and her AK-47 assault rifle. She's also developed a love of gunfights.
"I know I wasn't born to live forever. So when there's a fight I move forward and blast away. That way my nerves disappear. I'm always careful though to save one or two magazines for the retreat in case there's any trouble," Eliana explained.
That bristling confidence evaporates into a girlish giggle when I ask her if she's ever heard of Facebook or Twitter.
"I have no idea what Facebook is and I've never heard of Twitter," she said.
One morning during a break in the march, company commander Jagwin explained the FARC has managed to keep arms supply routes open.
He was sporting a new assault rifle he said was a South Korean-made version of the US military's M-16.
That, he said, had been smuggled into Colombia in oil barrels – the cost some 17 million pesos ($10,000).
The Russian-made PKM machine-gun used in the firefight at El Porvenir was also new.
And the previous afternoon, Jagwin had taken delivery of 100 rounds for a multiple grenade launcher. Price tag - 140,000 pesos ($83) each, he said.
All the ammunition appeared to be stamped with the brand and serial numbers of Indumil, the state-run munitions factory.
Getting hold of 81mm mortar bombs was proving a little more problematic, he confessed. A weapons smuggler was demanding 500,000 pesos ($295) each, he said.
As the latest rainstorm subsided, Jagwin issued orders to a four-man unit.
Scouts indicated the army was once again close by and he had decided to employ what is probably the most controversial weapon in the FARC's armory – homemade landmines.
"We use these mines to slow the enemy advance. We place them in the path of the army. Once they've passed we go and collect them again," Jagwin said.
Prior to the year 2000, all sides in Colombia's conflict used anti-personnel mines.
Since the Colombian government ratified the Ottawa Convention to ban landmines in 2000, the main culprits have been the FARC.
Colombia's legacy is bloody with one of the highest number of landmine victims after Afghanistan.
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According to Colombian government figures, more than 7,000 people were mutilated by mines between 1990 and 2010 - and around 2,000 were killed. Of the total 9,000 victims, about two-thirds were military personnel and the remaining third were civilians.
A young fighter spread around 20 mines on the grass – each with a diameter of about eight centimeters, made from a short section of PVC piping and packed with explosives.
"Take the GPS and mark where you’re placing the mines. Give them each a name, for example, Flower One, Flower Two etc so you can find them again," Willinton instructed as he helped prime the devices.
"We tell the peasants they should not move around after a certain time of day or in certain areas," Jagwin said.
It's a warning that is clearly far from effective. In the first two months of this year, 23 civilians fell victim to landmines.
Hearts and minds
The company's destination was a wooden shack nestled in the jungle. Pasted on the wall was a hand-written poster with the words "Jorge Briceño Civic-Military Brigade".
Guerrillas from a sister unit had set up a clinic to offer dental treatment and minor surgeries to peasant farmers and their families.
Plastic camouflage ponchos formed walls around the dental surgery. Another poncho marked the entrance to a side room where FARC medics set up, ready to operate using local anesthesia.
A civilian mother brought her three children along. Her previous attempt to get them treated by a civilian dentist in the town of La Julia, about three hours away, was a wasted journey.
"I took them to town but the nurse who pulls out teeth was not there that day so I had to bring them home," said the woman. She, like others at the FARC clinic, said dental treatment in town is free but poor quality under a government-subsidised health scheme.
While rebel clinics like these may be a useful stopgap measure for poor peasants, they are unlikely to be a comprehensive long-term solution to the precarious health conditions of Colombia's isolated rural communities.
Clearly this is a campaign by the FARC to win civilian "hearts and minds".
"What we're always looking to do is to win the support of the masses. Whoever wins the masses, wins the war. We also do this because we're working for the people," said Yesid, one of the rebel medics.
It’s a time-honoured tactic by any military force, especially those engaged in guerrilla warfare.
The clinic had already been operating for around an hour that day and a dozen civilians had gathered. Then news came that the army was approaching - treatment would have to be suspended immediately.
Art of war
The predicted clash with advancing army troops never took place. Guerrilla scouts had no clear idea of how many soldiers there were or their exact route.
Instead, the Marquetalia Company opted to move camp and outmanoeuvre its opponents.
"In a guerrilla war you choose where you will fight. We decide we can fight here or not, or decide to ambush them somewhere else," said Jagwin, expounding one of the key tenets of the art of guerrilla war. It's a sign this low-tempo conflict could drag on indefinitely, at least here in the countryside.
A day later the guerrilla clinic was back in business, several kilometers away.
By early morning there was already a list of 17 adults and children hoping for treatment.
There's no electricity in this region. Only a lucky few have portable generators or a solar panel.
So seeing neighbours having teeth pulled or getting sliced open for minor surgeries proved to be as much an attraction as TV for patients and idle onlookers alike.
A little girl watched her neighbour, "Don Luis", a peasant farmer, getting a hernia fixed.
Shafts of light streamed through the wood board walls. Medic Yesid and his three assistants worked under the light from battery-powered headlamps. The operating table was a wooden board partly propped up on a tree stump.
Once surgery started, the medics had no option except to continue – even if the army staged a surprise attack.
"If bombs start to fall or bullets start to fly we don't have much choice. We have to finish the job because we can't just leave the patient cut open on the operating table," medic Yesid said.
In an adjacent room, dentist Marta pulled teeth and replaced fillings. She has been in the FARC for 19 years and, like many others here, said she had joined up when she was just a child.
"I only studied until third grade in primary school. My mum abandoned us when I was just six and she left us with an alcoholic uncle," Marta explained.
"I used to sell ice-creams in the wholesale food market and scrounged for food for my brother. Then I went to work with my brother in Meta province on a farm - and that's where I came into contact with the guerrillas," she said.
Marta said she dreamed of becoming a dentist in civilian life when the conflict ended.
But she is adamant she will only lay down her weapon once the FARC takes power. That may seem like a hopeless illusion to the Colombian government and political analysts.
But Marta and her comrades cannot contemplate any other option.
"Some day this has to come to an end. Maybe I won't see live to see that day but it will come," she said. "I can't believe my struggle has been in vain."
Follow Karl Penhaul on Twitter: @karlpenhaul