* Security forces kill three, but truce generally holds
* U.N. set to vote on ceasefire observer mission
* Syria urged to implement U.N.-backed peace plan
BEIRUT, April 13 (Reuters) - Syrian forces loyal to President Bashar al-Assad killed at least three people as the opposition tried to stage protests after Friday prayers, marring a ceasefire brokered by U.N.-Arab League peace envoy Kofi Annan.
Syrians took to the streets across the country in small demonstrations after prayers, trusting that the truce, now in its second day, would protect them from the army bullets that have frightened off peaceful protesters for months.
Security forces shot dead at least one person in Hama as demonstrators tried to converge on a central square, the British-based Syrian Observatory for Human Rights said.
In the southern province of Deraa, where the uprising began, a man was shot dead as worshippers left a mosque in the town of Nawa. A third man was killed by security forces in the town of Salqeen in the northwestern province of Idlib, activists said.
Syria's state news agency SANA blamed two of the deaths on the opposition, saying an "armed terrorist group" shot dead the man in Salqeen and attributing the death of the Hama protester to a shot fired by a fellow demonstrator.
It also said "terrorists" shot dead an army major as he drove to work at his army unit, saying armed groups were seeking to "destroy any effort to find a political solution to the crisis" in Syria.
There were few reports of violence elsewhere, but activists said security forces were out in strength to prevent any major anti-Assad rallies around the country.
Assad's opponents had called for mass demonstrations to test whether the authorities would tolerate a return to peaceful protests, as Annan's six-point peace plan said they should.
But rallies videoed by activists were far smaller than the huge, chanting crowds seen in major cities at the start of the uprising 13 months ago and on several occasions in 2011.
International pressure has grown for Syria to fulfil all its commitments to the former U.N. chief by withdrawing troops and heavy weapons, permitting humanitarian and media access, releasing prisoners and discussing a political transition.
French President Nicolas Sarkozy said Assad's ceasefire declaration was insincere and renewed a call for the creation of aid passages, without saying how these could be protected.
"I firmly believe the international community should live up to its responsibilities and create the conditions for humanitarian corridors so that these poor people who are being massacred can escape a dictator," he told TV channel i>tele.
Sarkozy said he had discussed Syria with U.S. President Barack Obama on Thursday, including a plan to send in U.N. observers to verify compliance with the ceasefire between the military and rebels that came into effect at dawn that day.
The U.N. Security Council was to vote on the mission later on Friday, with an advance party of 10 to 12 expected to reach Syria next week before the deployment of up to 250 monitors.
Annan's spokesman Ahmad Fawzi would not name contributing countries, but told a news briefing in Geneva that many already had staff in the region, who could move swiftly.
Syrian Foreign Ministry spokesman Jihad Makdissi said a U.N. technical team that was in Damascus last week would return to the capital on Friday for more talks on a protocol under which the observer mission could be deployed.
A U.S.-drafted resolution threatened possible future action against Syria, saying the council "expresses its determination, in the event that the Syrian government does not implement its commitments, to consider further measures as appropriate".
Fawzi urged action on the other points of Annan's plan, especially the withdrawal of Syrian armed forces from populated areas, a condition that Syria was supposed to meet in the 48 hours leading up to the ceasefire deadline.
"We are worried about the operational deployment of heavy armour in population centres," he said. "They didn't belong there in the first place and they don't belong there now."
"We are thankful that there's no heavy shelling, that the number of casualties are dropping, that the number of refugees crossing the border are also dropping."
"BACK INTO THE ABYSS"
Underlining the fragility of the truce, Fawzi confirmed there had been some clashes. "Sometimes in situations like this the parties test each other... one shot, one bullet, can plunge Syria back into the abyss," he said.
Russia, which along with China has protected Syria from Security Council action, said rebels as well as government forces should be encouraged to comply with the ceasefire.
"We agreed to strengthen this new situation," Foreign Minister Sergei Lavrov said of talks he held with his U.S. counterpart Hillary Clinton in Washington on Thursday.
"Hillary Clinton agreed that alongside work with the Syrian government - and we essentially conduct this work daily - it is important to send similar signals to the armed opposition."
The British-based Observatory said fighting had erupted briefly in the northwestern province of Idlib, near the border with Turkey, after troops went to clear rebels from the area. There were no immediate reports of casualties.
The group's director, Rami Abdulrahman, said the truce was broadly holding, although four people were killed on Thursday and there was still no sign of any army withdrawal.
The United Nations estimates that Assad's forces have killed more than 9,000 people since the uprising began. Authorities blame the violence on foreign-backed militants who they say have killed more than 2,500 soldiers and police.