Activists in Syria say rebel forces are mounting a major offensive to try to capture the Taftanaz airbase in the north west of the country from government forces.

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The activist group the Syrian Revolution General Commission said they had begun "to liberate the Taftanaz airbase" from government control in Syria.

Rebels have been targeting air bases after Bashar al-Assad's regime increased the use of air strikes recently in a bid to stop rebel victories, and win back areas lost to their ground troops.

The Syrian Observatory for Human Rights, a pro-rebel group in Britain, said that heavy fighting was taking place near the airport in the Idlib province.

Video shooting controversy

The fighting follows the release of a video on Friday that appears to show rebel soldiers killing captured government forces that had surrendered.

The solders are seen to call the men "Assad's dogs" before repeatedly shooting them as they lay on the ground.

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The UN has said the video probably constitutes a war crime and that it should be investigated.

"It will be examined carefully," said a UN spokesman Rupert Colville.

He told a news briefing in Geneva, adding that may be difficult to identify the perpetrators and the location where the footage was recorded.

"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.

He added: "A year ago, reports of excesses and atrocities by opposition forces were few and far between. But towards the end of last year and increasingly during 2012, we've seen events like this happen."

The footage has dealt a blow to the image of the rebels, just before a major conference in Qatar where the United States is trying to unify the opposition groups into a new leadership.

The west has held back from supporting the rebellion more strongly because of fears over the multiple divisions of the rebel forces and their lack of organisation.

There have been divisions between Islamist and secularist groups, and also between those operating inside and outside of the war-torn nation. The United States has expressed its desire more control is taken by those operating inside Syria, rather than the current main group - the Syrian National Council, which is based outside the country.

More than 36,000 people have been killed since the uprisings began against President Bashar al-Assad in March 2011.

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