A blast hit troops escorting UN observers in Syria's restive south on Wednesday, a day after envoy Kofi Annan warned his peace plan could be the last chance to avoid civil war.

The explosive device, which appeared to have been planted underground, detonated as the convoy of four vehicles entered the town of Daraa, cradle of a 14-month uprising against President Bashar al-Assad's regime.

Major General Robert Mood of Norway, the head of the UN mission, was in the convoy but escaped unharmed along with 11 other observers and his spokesman Neeraj Singh, said the photographer who was travelling in the convoy.

The bomb attack was the latest breach of a month-old ceasefire agreement that UN-Arab League envoy Annan said could be the last chance to avert a civil war in Syria.

Opposition bloc the Syrian National Council accused the regime of being behind the blast.

"We believe the regime is using these tactics to try to push the observers out amid popular demands to increase their numbers," SNC executive committee member Samir Nashar said.

Troops elsewhere pounded a rebel hideout near Damascus, said the Syrian Observatory for Human Rights.

The shelling of Douma, about 13 kilometres (eight miles) northeast of the capital, came as violence across Syria killed at least four people, among them three soldiers and one civilian, said the Britain-based watchdog.

In the village of Manshiya, near Daraa, several soldiers were killed and wounded when an explosion targeted their vehicle, although the exact number was unclear.

"We know there were casualties, but we do not know how many," the Observatory's Rami Abdel Rahman said.

Clashes between regime forces and armed rebel groups in the village of Marata village, northwestern Idlib province, killed one soldier as he tried to flee the shooting, said the watchdog.

Security forces carried out arrest raids in Harasta, outside Damascus, and the villages of Al-Safira and Al-Hisan, in eastern Deir Ezzor province.

Two security forces members were killed soon after midnight in the Jura district of Deir Ezzor, the scene of heavy shooting and explosions, said the Observatory.

One civilian was killed and three wounded in heavy machinegun fire by regime forces in Tell Ain al-Hamra, near the town of Jisr al-Shughur in the northwestern province of Idlib.

On Tuesday, Annan told the UN Security Council the priority in Syria was "to stop the killing," and expressed concern that torture, mass arrests and other human rights violations were intensifying.

Regime forces "continue to press against the population," despite a putative truce that started on April 12, but attacks are more discreet because of the presence of the UN military observers, diplomats quoted him as saying.

"The biggest priority, first of all we need to stop the killing," Annan told reporters in Geneva, adding that his six-point peace plan is "the only remaining chance to stabilise the country."

Annan briefed the council on his efforts to get Assad to implement the plan, which he said was possibly "the last chance to avoid civil war."

He stressed, however, that the peace bid was not an "open-ended" opportunity for Assad, the diplomats who attended the briefing said.

Annan plans to return to Damascus in the coming weeks, his spokesman said, though this depended on events on the ground there. It would be only his second visit since his mission began earlier this year.

US ambassador to the UN Susan Rice said Washington's goal was still the removal of Assad.

"The United States remains focused on increasing the pressure on the Assad regime and on Assad himself to step down," Rice said.

"The situation in Syria remains dire, especially for the millions who continue to endure daily attacks and are in urgent need of humanitarian assistance," she told reporters after Annan's briefing.

Annan updated the UN body on the status of his six-point plan after UN chief Ban Ki-moon warned world powers were racing against time to prevent all-out civil war.

The current 60 or so observers on the ground "have had a calming effect" and the deployment by the end of the month of a 300-strong team would see a "much greater impact," Annan said.

The Observatory said Tuesday that almost 12,000 people, most of them civilians, had been killed since the revolt broke out in March 2011. About 800 of them had died since the truce was supposed to have taken effect.

Turkish Prime Minister Recep Tayyip Erdogan urged the UN to bolster its observer mission well past the 300 authorised under a Security Council resolution.

The United Nations has accused both the Syrian regime and rebels of violating the truce.