The UN has warned that violence in Syria is getting worse as the military steps up assaults on civilian centres and the opposition increasingly turns to co-ordinated attacks on government forces.
"The secretary-general expresses his deep concern at the dangerous intensification of armed violence across Syria over the past several days, and the grave danger facing civilians in areas under fire," Ban Ki-moon's office said in a statement. "The bloodshed and fighting must stop at once."
It said unarmed UN observers in Syria have reported an increased level of armed clashes between government and rebel forces.
The UN observer force, known as UNSMIS, has also been observing "planned and co-ordinated attacks [by rebels] on
government forces and civilian infrastructure in multiple locations".
Violence in Syria has spiked in recent weeks, as both sides ignore a peace plan crafted by UN-Arab league envoy Kofi Annan.
In the US, state department spokeswoman Victoria Nuland voiced fears that the Syrian government was planning a "massacre" in the opposition stronghold of Haffeh, near Turkey.
She said Washington shared the alarm of Annan, who described reports of mortar bombs, helicopters and tanks being used in Haffeh, as well as government shelling of the city of Homs on Monday.
"We are calling this out now in the hope that we can stop what could be a potential massacre," Nuland said, citing reports from UN monitors on the ground.
Nuland said Syrian President Bashar al-Assad's forces were using "horrific tactics" including firing on civilians from airborne helicopters and deploying plainclothes "shabiha" militia, which the opposition has accused of slaughtering civilians in earlier attacks in the villages of Houla and al-Qubayr.
"This constitutes a very serious escalation," Nuland said.
Clashes in al-Haffeh
Ban demanded UN access to the Syrian city of al-Haffeh, a town of about 30,000 near the Mediterranean coast.
"Rarely, have I seen such brutality against children as in Syria, where girls and boys are detained, tortured, executed, and used as human shields "
- Radhika Coomaraswamy, UN special representative
Fighters in the town said on Tuesday that hundreds of rebels were fighting a tank and helicopter-backed assault on their district, tucked among rugged mountains.
Clashes started a week ago between rebels and security forces who were setting up checkpoints to tighten their grip on the strategic town, which lies close to the port city of Latakia as well as the Turkish border. It has allegedly been used by rebels to smuggle people and supplies.
The state-run SANA news agency said on Tuesday that authorities were continuing "persuasion of the remnants of the terrorists who committed vandalism acts and sabotaged public and private properties in al-Haffeh"..
SANA quoted an "official source" saying "a number of terrorists were killed and others wounded in the process" and "a number of terrorists" apprehended.
The UN mission also expressed concern over the situation in the flashpoint city of Homs, saying it had received reports of "a large number of civilians, including women and children trapped inside the town and are trying to mediate their evacuation".
Al-Khaldiyeh, al-Qusour and Juret al-Shayyah neighbourhoods were under intense shelling for a fourth day on Tuesday, according to activists, who said Red Crescent teams and other medics were denied access to treat the injured.
Activists said the Free Syrian Army was preventing the army from storming the areas to protect civilians.
"We think any area stormed by the Syrian army will witness massacres," activist Hadi al-Abdallah told Al Jazeera.
He said the opposition strongholds of Rastan, Talbiseh and al-Qusayr near Homs were also under attack.
Peace plan 'abandoned'
The bloodshed in the country has persisted despite the presence of 300 UN observers charged with monitoring both sides' compliance with Annan's peace plan, which calls for an end to all violence and an inclusive, Syrian-led political transition.
"If you talk to the opposition, they say the observers have not done anything," Al Jazeera's Rula Amin, reporting from neighbouring Lebanon, said." "That's why they have not abided by this plan, saying they cannot lay down their weapons and have to defend themselves.
"If you talk to government sources, they will say that the presence of observers give enough cover to the armed groups to regroup, to restructure, and be able to carry out deadly attacks against government security forces. It's obvious both sides have given up on this plan."
The opposition says about 13,000 people have been killed since the uprising against Assad began in March 2011. Authorities say more than 2,600 members of the security forces have been killed.
Most accounts of violence cannot be independently verified, as Syria restricts access for journalists.
Rights groups say about 1,200 of the dead are children.
Children as 'human shields'
In a UN report released on Tuesday, Syrian troops were accused of torturing and executing children, and of using children as young as eight as "human shields" during military raids against rebels.
In-depth coverage of escalating violence across Syria
The UN Nations branded the Syrian government as one of the worst offenders on its annual "list of shame" of conflict countries where children are killed, tortured and forced to fight.
"I have never seen such terrible action against children," Radhika Coomaraswamy, UN special representative for children in armed conflict, told Al Jazeera. "Actually torturing children, putting children on tanks, using them as human shields, or summarily executing children - these are things that normally don't happen in warfare."
Government forces rounded up dozens of boys aged eight to 13 before an attack on the village of Ayn l'Arouz in Idlib province on March 9, the report said.
The children were "used by soldiers and militia members as human shields, placing them in front of the windows of buses carrying military personnel into the raid on the village," it said.
Quoting witnesses, the UN report said Syrian military and intelligence forces, as well as pro-government Shabiha militiamen, surrounded the village for an attack that lasted more than four days.
Among the 11 dead on the first day were three boys aged 15 to 17. Another 34 people, including two boys aged 14 and 16 and a nine-year-old girl, were detained.
"Eventually, the village was reportedly left burned and four out of the 34 detainees were shot and burned, including the two boys aged 14 and 16 years," the Children in Armed Conflict report said.
The report also said the UN had received "some credible allegations" of the recruitment and use of children by the armed opposition, including the Free Syrian Army, which has a stated policy of not recruiting any child under 17 years of age.
"Of course at the command level, they deny it, and I think they don't have any policy of recruiting children. But on the field level, it is taking place," Coomaraswamy said.