Syrian rebels have launched a major assault on a northern air force base, on the eve of a crucial meeting in Qatar to discuss the future of the opposition.
Saturday's attack on the Taftanaz base, from where helicopter gunships raid opposition positions and rebel-held areas, comes after troops launched an unprecedented wave of air strikes this week in a bid to reverse rebel gains.
A video posted on the internet said eight battalions were taking part in the attack, including the Islamist Al-Nusra Front, and showed a missile launcher mounted on the back of a pick-up truck firing on regime positions.
The Syrian Revolution General Commission, a network of local activists, said an operation had begun "to liberate the Taftanaz airbase".
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The Syrian Observatory for Human Rights, a Britain-based rights watchdog, said "heavy fighting" had broken out near the base in Idlib province, where rebels have seized new ground this week.
"The rebels' gains in the north seem irreversible," Thomas Pierret, a Syria expert at the University of Edinburgh's Islamic and Middle Eastern Studies department, said.
He said regime forces appeared to be concentrating their efforts in the region on defending the embattled commercial hub Aleppo, which rebel advances in the past month have cut off from Damascus and the Mediterranean coast.
"The problem with this strategy is that the Aleppo garrisons are now largely isolated. It is likely they will fall in the months to come," he said.
The fresh clashes came as the opposition prepared for key talks starting Sunday in Qatar, where the US is expected to push for a new umbrella organisation to unite the country's fractured regime opponents.
Reports have emerged that Washington will press for an overhaul of the opposition and its main body, the Syrian National Council (SNC), with long-time dissident Riad Seif touted as the potential head of a new government-in-exile dubbed the Syrian National Initiative.
In a statement on Saturday, a group of opposition figures, including Seif, sought to quell concerns the overhaul is aimed at building an opposition that would be willing to negotiate with President Bashar al-Assad.
"Assad and his entourage leaving power is a non-negotiable precondition for any dialogue aimed at finding a non-military solution, if that is still possible," the group said in the statement, after talks in Amman.
The meeting in the Jordanian capital also came out in support of "efforts underway to put in place a unified political body for the whole of the opposition".
The SNC lashed out at alleged US interference with the opposition on Friday, accusing Washington of undermining the country's revolt and "sowing the seeds of division" by seeking the overhaul.
US Secretary of State Hillary Clinton this week voiced frustration with the SNC, saying it was not representative of on-the-ground opposition forces and that it "can no longer be viewed as the visible leader of the opposition".
The observatory, meanwhile, released new videos of pro-regime fighters apparently killing prisoners and cutting ears from bodies, after footage showing rebels executing soldiers had raised international concerns on Friday.
The purported video of the rebels - also released by the observatory - showed about 10 soldiers being beaten, then lined up on the ground and executed with automatic rifles.
The UN human rights body said the video appeared to show a war crime and warned that "accountability will follow" for those who commit atrocities, while London, Paris and Washington raised concerns.
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One of the two new videos released on Saturday, reportedly filmed in July in the northwestern Latakia region, shows a man in military fatigues brandishing a severed ear and a knife, laughing at the camera.
"Here is the ear of a dog - we will teach them a lesson," he says, referring to the rebels.
The other, reportedly filmed in February in the southern Daraa province, shows fighters using automatic weapons to execute a group of rebels lying on the ground. The authenticity of the videos could not be verified.
The observatory said at least 120 people were killed in violence on Saturday. Among them were said to be 54 civilians, including eight children and four women, who died in an air raid on the town of Zamalka just east of Damascus.
The watchdog says more than 36,000 people have died since the uprising against Assad's rule broke out in March 2011, first as a protest movement inspired by the Arab Spring, and then as an armed rebellion after brutal repression by the regime.
In other developments, Israel complained to the UN on Saturday after three Syrian tanks breached the demilitarised Golan Heights zone that separates the two countries, a military spokeswoman said.