Internet video posted by Syrian rebels appears to show fighters loyal to President Bashar al-Assad stabbing two men to death and stoning them with concrete blocks in a summary execution lasting several minutes.

Assad's forces and rebels have both been accused of atrocities in Syria's 21-month-old uprising-turned-civil war. The United Nations says the authorities and their allied militia have been more culpable.

Reuters could not verify the provenance of the footage or the identity of the perpetrators and their victims. The video was posted on Tuesday but it was not clear where or when it was filmed. However it does clearly show a summary execution and torture, apparently being carried out by government supporters.

At one point, one of the perpetrators says: "For God's eyes and your Lord, O Bashar," an Arabic incantation suggesting actions being carried out in the leader's name.

The video was posted on YouTube by the media office of the Damascus-based rebel First Brigade, which said it had been taken from a captured member of the shabbiha, a pro-government militia made up mainly of members of Assad's minority Alawite sect.

International media have often used videos provided daily by rebels even though it is difficult and frequently impossible to verify their contents. The government tightly restricts media access in Syria, where 28 journalists were killed last year.

The graphic video of killings begins with a group of captive men huddled against a wall in a room filled with broken masonry. Most wear jeans and t-shirts pulled up over their heads, covering their faces.

Five armed men in camouflage uniforms and holding assault rifles stand behind them, one of whom turns to the camera to grin and wave. Several captives are led out the room, leaving two men huddled against the wall.

A tall, bearded soldier with dark sunglasses and a black Nike baseball cap lightly slashes at the men's backs with a knife. Two others hand their guns to their comrades and unsheathe knives.

One of them, a shorter man with a beige cap, turns to the camera and waves his knife with a flourish. The soldiers slash and then start to stab the captives, who shake and groan.

A close-up shows cuts through one victim's blue sweater to his bare skin. Blood starts to flow down his back. The other victim falls to the ground.

The soldiers show off for the camera, smiling for close-up shots and slicing at the men's backs. They start stabbing the men more deeply and violently in the back and side. The captives fall to the ground.

One of the stabbed men is still moving when a soldier throws a large piece of broken masonry on his head. Others follow suit until the men are almost completely covered in concrete bricks, now stained with their blood.

The soldiers walk away and one slaps his hands together to remove the dust.

An estimated 45,000 people have been killed during the conflict in Syria.

(Here is a link to the video. Warning: graphic content)

(Reporting by Sara Elizabeth Williams and Oliver Holmes; Editing by Peter Graff and Alistair Lyon)