SYRIAN rebels say they have launched a major offensive to take a key airbase in the north of the country.

A video posted on the internet showed fighters attacking the base in Taftanaz in an attempt to control a strategic zone between Syria's two biggest cities, Aleppo and Damascus.

It comes just as the country's fragmented opposition groups meet in Doha, Qatar today to find a common position to increase international support against the regime of Syrian President Bashar al-Assad.

The Taftanaz attack took place at dawn with five units of rebel fighters opening fire with multiple rocket launchers, mortars and other weaponry, according to the video.

In recent months, government forces have increasingly used air power to strike areas held by the rebels, who lack anti-aircraft weapons to deter the attacks.

Rebel forces are reported to have taken control of the main roads in much of the area southwest of Aleppo.

The video said the rebel groups involved included several brigades of the Free Syrian Army, but also the radical Islamist al-Nusra Front.

The event follows another video released online showing the apparent murder of a number of Syrian troops who had surrendered to a rebel group.

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

It appears from the footage that some of those who died were shot after they had surrendered.

Rebels apparently berated them, calling them "Assad's dogs" in reference to President Bashar al-Assad, before firing round after round into their bodies as they lay on the ground.

The UN human rights office said the video, which has drawn international condemnation, must be verified but the actions seen in it appear to constitute a war crime.

Spokesman Rupert Colville said: "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.

"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.

"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]."

On Friday, the US said it "condemned human rights violations by any party in Syria".

State department spokeswoman Victoria Nuland said: "There is no justification for that kind of behaviour, ever. Anyone committing atrocities should be held to account."

The question of apparent brutality by some rebel units has become a serious concern ahead of the major opposition meeting being held in Qatar.

The US is hoping a new leadership will help unify the disparate opposition elements and bring a successful conclusion to the long-running uprising. Anti-regime activists say more than 35,000 people have been killed since the uprising against Assad started in March 2011.

Divisions have arisen not just between Islamist and secularist groups, but also between those operating inside Syria and opposition figures working abroad.

Nuland said there was a need for cohesion of rebel forces within Syria and a strong organisation outside the country that could work with the international community.

A previous opposition meeting in Cairo in July accepted that the Assad government must fall, but failed to appoint a committee to act for the opposition internationally.

The US this week signalled the opposition needed to be expanded from the main overseas group, the Syrian National Council, to take in more of those operating inside Syria. Representatives at Doha will include various other religious and secular groupings, alongside Kurdish figures and dissident members of Assad's Alawite sect.

■ syria