REBELS seized control of one of Syria’s largest helicopter bases on Friday, opposition sources said, in their first capture of a military airfield used by President Bashar al-Assad’s forces.

Fighting raged across the country as international mediator Lakhdar Brahimi sought a political solution to Syria’s civil war, meeting senior U.S. and Russian officials in Geneva.

But the two world powers are still deadlocked over Assad’s fate in any transition.

The United States, which backs the 21-month-old revolt, says Assad can play no future role, while Syria’s main arms supplier Russia said before the talks that his exit should not be a precondition for negotiations.

Syria is mired in bloodshed that has cost more than 60,000 lives and displaced millions of people. Severe winter weather is compounding their misery. The U.N. children’s agency UNICEF says more than 2 million children are struggling to stay warm.

The capture of Taftanaz air base, after months of sporadic fighting, could help rebels solidify their hold on northern Syria, according to Rami Abdelrahman, head of the pro-opposition Syrian Observatory for Human Rights.

But Yezid Sayigh, at the Carnegie Middle East Centre in Beirut, said it was not a game-changer, noting that it had taken months for the rebels to overrun a base whose usefulness to the military was already compromised by the clashes around it.

“This is a tactical rather than a strategic gain,” he said.

In Geneva, U.N.-Arab League envoy Brahimi’s closed-door talks began with individual meetings with U.S. Deputy Secretary of State William Burns and Russian Deputy Foreign Minister Mikhail Bogdanov. He later held talks with both sides together.

A U.S. official said negotiations would focus on “creating the conditions to advance a political solution - specifically a transitional governing body”.