A video appearing to show Syrian rebels killing soldiers who had surrendered must be verified but appears to constitute a war crime that should be prosecuted, the United Nations human rights office has said.

UN human rights spokesman Rupert Colville told a news briefing that it was hard to immediately verify the details of the video but it did appear to be a war crime.

"Like other videos of this sort, it's difficult to verify immediately in terms of location, who's involved. We need to examine this carefully. It will be examined carefully," he said.

"But the allegations are that these were soldiers who were no longer combatants. And therefore, at this point it looks very likely that this is a war crime, another one," he said.

Anti-government rebels killed 28 soldiers yesterday in attacks on three army checkpoints around Saraqeb, a town on Syria's main north-south highway, a monitoring group said.

Some of the dead were shot after they had surrendered, according to video footage.

Rebels berated them, calling them "Assad's dogs", before firing round after round into their bodies as they lay on the ground.

"Unfortunately, this could be just the latest in a string of documented summary executions by opposition factions as well as by government forces and groups affiliated with them, such as the shabbiha [pro-government militia]," Mr Colville said.

Meanwhile, Syrian government war planes have intensified air strikes on suburbs of Damascus that are under rebel control.

Footage posted by the opposition shows scenes of carnage, with mangled bodies and houses reduced to rubble.

The strikes on Damascus are reported to be focused on the densely populated Sunni Muslim areas known as the Eastern Ghouta, a stronghold of rebels fighting to overthrow President Bashar al-Assad.

Activists say that while the air raids target the rebels, they also aim to inflict maximum damage to drive a wedge between the opposition and the civilian population.

State media has said the military is "cleansing" what it describes as terrorists from the area.