BEIRUT – Hundreds of Islamic militants fighting to topple Syrian President Bashar Assad seized parts of a strategic northwestern air base Thursday after weeks of battling government troops for control of the sprawling facility.

At stake is the biggest field for helicopters used to bomb rebel-held areas in the north and deliver supplies for regime forces.

Opposition fighters and activists said rebels broke into Taftanaz air base in the northern Idlib province Wednesday night and by Thursday had seized control of more than half of it. Intense battles were still raging, and one activist said rebels had suffered losses.

An activist near there Thursday evening said the government bombed the base from warplanes to push back rebels who seized several helicopters. The account from the activist, who requested anonymity out of fear of reprisals, could not immediately be confirmed.

An amateur video posted by activists online showed smoke rising from behind helicopters parked at the Taftanaz tarmac, and a narrator said it was the result of an airstrike. The video appeared consistent with Associated Press reporting.

Meanwhile, Iran’s official IRNA news agency said 48 former captives held for more than five months returned on Thursday to Tehran after being freed by Syrian rebels in the first major prisoner swap of the civil war.

The rebel attack on the Taftanaz base is part of a wider attempt to chip away at the Syrian regime’s air supremacy, which poses the biggest obstacle to the opposition fighters’ advances.

“If the fighters seize full control of Taftanaz air base and manage to keep it, it would be the first major military airport to fall into rebel hands,” said Rami Abdul-Rahman, director of the Britain-based Syrian Observatory for Human Rights.

Abdul-Rahman, who relies on a network of activists on the ground in Syria, said the rebels involved in the attack on Taftanaz were Islamic militants. He said estimated their number to be about 700 and said they included members of Jabhat al-Nusra, affiliated with al-Qaida, and groups with a similar Islamic ideology.

Also Thursday, Syria accused the joint U.N.-Arab League peace envoy of “flagrant bias,” further complicating diplomatic efforts to end the bloodshed.

In a strongly worded statement, Syria’s Foreign Ministry criticized international envoy Lakhdar Brahimi for comments he made to the media, saying he has “diverted from the core of his mission” in favor of those who “conspire” against Syria.

At least 60,000 people have been killed in the war, according to a recent U.N. estimate, and all international efforts so far to bring about a peaceful transition have failed.