Violence in Syria killed four civilians on Saturday, one of them a woman, a rights watchdog said, even as a UN mission charged with overseeing a battered truce neared half its planned strength.

In Idlib province, a stronghold of rebels fighting President Bahar al-Assad's regime, security force gunfire killed a man and a woman during a series of raids, the Syrian Observatory for Human Rights said.

A third civilian was killed during pre-dawn shelling of the village of Mork in central Hama province, the Britain-based watchdog said, while a fourth was killed by sniper fire in the northeastern city of Deir Ezzor.

Troops clashed with rebel fighters in Idlib, in the flashpoint central province of Homs, in southern Daraa province, and in several areas of Damascus province, the Observatory said, without any immediate word of casualties.

The persistent violence came as the UN mission in Syria said it now had 145 military observers on the ground, just shy of half the force of 300 authorised by the Security Council. They are backed by 56 civilian staff.

The observers are tasked with shoring up a promised ceasefire brokered by UN-Arab League peace envoy Kofi Annan that was supposed to take effect on April 12 but which has been broken daily.

On Thursday, twin suicide bombings in the capital killed at least 55 people and wounded 372 -- the deadliest attacks since the uprising against Assad's regime erupted in March last year.

State television said the army foiled an attempt to carry out a similar suicide bombing in Syria's second-largest city Aleppo on Friday.

Both Damascus and Aleppo have been hit by a spate of bombings since December 2011 but the attacks have escalated since the UN mission deployed.

State media have accused the West and its regional allies of opening the door to Al-Qaeda through its backing of the opposition and state television has played heavily on the fears of the public.

The broadcaster has repeatedly aired graphic images of the dead and wounded in Thursday's bombings, alongside interviews with residents and nationalist slogans.

"Syria loves life" flashes across the screen, followed by a clip from the interior ministry in which an eagle flies towards a Syrian flag.

Photographs of gas cylinders and other makeshift explosive devices are shown, followed by a number for citizens to call if they have information.

The main opposition coalition, the Syrian National Council, has accused the authorities of resorting to the "terrorism" itself in a bid to torpedo Annan's peace plan.

According to the Observatory, another 17 people were killed in violence on Friday, including 12 civilians, while the total death toll has climbed to more than 930, over two-thirds of them civilians, since the ceasefire.