He said that while there was mostly a tense quiet during the day, more clashes were likely after nightfall when darkness gives the protesters more opportunity to get close to the security forces and their checkpoints.
With tensions rising, the UN secretary general, Ban Ki-moon, has called on all sides to avoid violence and resolve the crisis through talks.
"He strongly encourages them to urgently return to dialogue in order to de-escalate the situation and resolve matters peacefully," Ban's spokesman said in a written statement.
The clashes came as the Thai government vowed it would restore order "in the next few days", following two months of red shirt street protests that have paralysed key parts of Bangkok.
Troops fired tear gas, rubber bullets and live rounds at defiant red shirt protesters who fought back with petrol bombs, stones and homemade rockets.
Some protesters set vehicles on fire and rolled burning tyres into checkpoints of troops.
Army spokesmen said security forces were concentrating their efforts on tackling a hardcore of armed "terrorists" they said had hidden themselves among the protesters.
With international calls growing for both sides to return to dialogue, Panitan Wattanaygorn, spokesman for the Thai government, told Al Jazeera that there would be no negotiations until the protests end.
"The position of the government is very clear," he said, adding that the main aim of the army operations was to secure the Rajprasong area and protect the wider public.
Al Jazeera's Aela Callan, reporting from Bangkok, said that the government spokesman's comments appeared to contradict television footage which appeared to show troops firing indiscriminately, including incidents in which some reporters covering the protests were fired upon.
"It's a very confusing situation," she said. "What we're going to have to watch for now is the reaction of the red shirts."
She said some red shirt sources in the northeast of Thailand had told her they were being prevented from sending bus loads of supporters to join the protests in Bangkok, while other more "hardline" red shirts were looking at more creative ways to bring reinforcements into the city.
"There are still thousands of people down there and there are still thousands that are willing to defend the barricades," she said.
Earlier, Sean Boonpracong, a red shirt spokesman, called on the army to end its operations against the protest camp saying the two sides were unmatched.
"We want the army to cease fire," he told Al Jazeera. "We really seriously want peace. We are really concerned at what is taking place."
Other red shirt leaders have said they believe the death toll from the violence is much higher than the 16 killed the government has reported.
Meanwhile Thaksin Shinawatra, the exiled former prime minister who is closely linked to the red shirt protesters, accused the Thai government of a "grave infringement of human rights".
In a Twitter message from exile, Thaksin said the "very cruel and unusual government" will "end up as war criminals" in the International Court of Justice.
|The government has vowed to restore order within "the next few days" [AFP]
Amid fierce clashes on Friday night, defiant red shirt leaders led followers in Buddhist prayers and called on volunteers to bring more tires to use as barricades around the camp.
"Death cannot stop us civilians from continuing our fight," Jatuporn Prompan, a protest leader, said.
The clashes first erupted late on Thursday after a suspended army general allied with the red shirt movement was left in a coma after being shot in the head.
Witnesses said the shooting was apparently carried out by a sniper, but it was not clear exactly who was behind the attack.
Panitan Wattanaygorn, the acting Thai government spokesman, told Al Jazeera that soldiers were not involved.
"Our operation is to secure the outer areas of the demonstrations. We will investigate as the red shirts have problems with their own leaders. Our officer was killed in a similar way in the last week," Panitan said.
But Phongthep Thepkanjana, a red shirt ally and former minister of justice, told Al Jazeera: "I don't think anybody can accept that because there was information that the government has snipers deployed close to the demonstration site. He was shot from a long distance."
Street rallies have been held since March 12 in an attempt to force the prime minister, Abhisit Vejjajiva, to dissolve parliament and call fresh elections.
The red shirts say Abhisit's government is illegitimate because it came to power in a 2008 parliamentary vote following the controversial court-ordered dissolution of the previous pro-Thaksin government.
Abhisit on Thursday said he was shelving a previously announced plan to hold early elections in November after demonstrators refused to abandon their Bangkok protest camp.