Syria's opposition is calling for widespread protests Friday to test the regime's commitment to an internationally brokered ceasefire that the United Nations chief described as so fragile it could collapse with a single gunshot.

Regime forces halted heavy shelling and other major attacks in line with the truce that began at dawn Thursday, though there were accusations of scattered violence by both sides. The government ignored demands to pull troops back to barracks, however, defying a key aspect of the plan, which aims to calm a year-old uprising that has killed 9,000 people and has pushed the country toward civil war.

"The onus is on the government of Syria to prove that their words will be matched by their deeds at this time," UN Secretary General Ban Ki-moon told reporters in Geneva. He said the world was watching with skeptical eyes.

"This ceasefire process is very fragile. It may be broken any time," Ban added, saying "another gunshot" could doom the truce.

The presence of tanks and troops could discourage any large gatherings, but the leader of the opposition Syrian National Council, Burhan Ghalioun, urged Syrians to demonstrate peacefully on Friday.

"Tomorrow, like every Friday, the Syrian people are called to demonstrate even more and put the regime in front of its responsibilities — put the international community in front of its responsibilities."

A massive protest would be an important test of the ceasefire — whether President Bashar al-Assad will allow his forces to hold their fire and risk ushering in a weeks-long sit-in or losing control over territory that government forces recently recovered from rebels.

So far, the military crackdown has prevented protesters from recreating the powerful displays of dissent seen in Egypt's Tahrir Square, where hundreds of thousands of people camped out in an extraordinary scene that drove longtime leader Hosni Mubarak from power.

If the truce holds, it would be the first time the regime has observed an internationally brokered ceasefire since Assad's regime launched a brutal crackdown 13 months ago on mass protests calling for his ouster.

An outbreak of violence at a chaotic rally could give the regime a pretext for ending the truce. And it would be difficult to determine the source of such an attack, given that Syria is largely sealed off from journalists and outside observers.

UN envoy urges observer mission

The UN chief's envoy, Kofi Annan, urged the 15-nation UN Security Council to authorize an observer mission that would keep the ceasefire going and to demand that Assad order his troops back to barracks, UN diplomats said. The council could adopt a resolution on the observers as early as Friday, the diplomats said on condition of anonymity because the meeting was closed.

Western powers, skeptical that Assad will call off the killings, said an end to violence is just the first step.

U.K. Prime Minister David Cameron urged Syria's allies Russia and China to help "tighten the noose" around Assad's regime. Russia and China have blocked strong action against Syria at the Security Council, fearing it would open the door to possible NATO airstrikes like those that helped topple Libya's Moammar Gadhafi.

U.S. Secretary of State Hillary Rodham Clinton noted that Assad failed to comply with key obligations, such as pulling back tanks.

"The burden of fully and visibly meeting all of these obligations continues to rest with the regime," she said. "They cannot pick and choose. For it to be meaningful, this apparent halt in violence must lead to a credible political process and a peaceful, inclusive democratic transition."

A Syrian anti-government activist in Homs poses as buildings damaged by the government army, according to the opposition, are seen in the background. (Shaam News Network/Reuters)

The U.S. Embassy in Damascus published an image on its Facebook page that purports to show tanks deployed within the city of Homs.

"Clearly, Assad is not complying," the embassy said.

Bassma Kodmani, a spokeswoman for the opposition Syrian National Council, said a heavy security presence, including checkpoints and snipers, remained in the streets despite the ceasefire.

"There is no evidence of any significant withdrawal," she told reporters in Geneva. "The real test for us today is if people can go and demonstrate peacefully" she added. "This is the real reality check."

In the hours after the 6 a.m. deadline, at least four civilians were reported killed — three of them by sniper fire — and the state-run news agency said "terrorist groups" set off a roadside bomb that killed a soldier. But there was no sign of the heavy shelling, rocket attacks and sniper fire that have become routine.

Troops also intensified searches at checkpoints, tightening controls ahead of possible large-scale protests Friday.