Syrian opposition forces launched on Saturday what they said was a major assault on a northern airbase used to deploy regime air power, on the eve of a crucial meeting to decide the future of the opposition.
The attack on the Taftanaz base, from where helicopter gunships raid opposition positions and rebel-held areas, comes after regime forces this week launched an unprecedented wave of air strikes in a bid to reverse rebel gains.
A video posted on the Internet said eight battalions were taking part in the attack, including the radical 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 activists on the ground, said an operation had begun "to liberate the Taftanaz airbase."
The Syrian Observatory for Human Rights, a Britain-based watchdog, said "heavy fighting" had broken out near the base in Idlib province, where rebels have seized new ground this week.
Analysts said the assault came as opposition forces clearly have the momentum in the battle for Syria's northwest.
"The rebels' gains in the north seem irreversible," said Thomas Pierret, a Syria expert at the University of Edinburgh's Islamic and Middle Eastern Studies department.
He said regime forces appeared to be concentrating their efforts in the region on defending 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.
A formerly imprisoned dissident, meanwhile, told Reuters that the fragmented Syrian opposition will attempt once again this weekend to forge a common policy to gain international respect, obtain weapons and, most importantly, topple Syrian President Bashar al-Assad.
"An alternative to the regime is dearly needed," said Riad Seif, a liberal politician who is battling cancer and managed to leave Syria only a few months ago after having been imprisoned.
"We are talking about a temporary period that begins with forming a political leadership until a national assembly that represents all Syrians meets in Damascus, once Assad falls," Seif said in an interview with Reuters in Amman.
He spoke after talking to opposition figures in advance of a of a meeting of the wider opposition movement in Doha this weekend to form a united front to help end the 19-month uprising against Assad that has claimed more than 32,000 lives, left many parts of the country in ruins and threatens to widen into a regional conflict.
Divisions between Islamists and secularists as well as between those inside Syria and opposition figures based abroad have thwarted prior attempts to forge a united opposition.
On Wednesday, the United States called for an overhaul of the Syrian opposition's leadership, saying it was time to move beyond the Syrian National Council (SNC), the largest of the groupings abroad, and bring in those "in the front lines fighting and dying".
Secretary of State Hillary Clinton, signaling a more active stance by Washington in attempts to form a credible political opposition to Assad, said the meeting in Qatar would be an opportunity to broaden the coalition against him.
Unlike previous efforts that failed to come up with a unified leadership, Seif said the Doha assembly will be more inclusive, representing a myriad of religious and activists' groupings as well as more members of Assad's minority Alawite sect and Kurdish political leaders.
Among those Seif met in Amman was former Syrian Prime Minister Riad Hijab, who defected to Jordan three months ago and is playing a major role in the new effort led by Seif.
He also met with Suhair al-Atassi, an organizer of peaceful street demonstrations early in the revolt, and physician Kamal al-Labwani, a long-time political prisoner who is now an outspoken advocate for armed struggle.