BEIRUT - A Syrian rebel group that the United States has labeled an affiliate of al-Qaida in Iraq appeared Thursday to be on the verge of overrunning a government air base that's used to launch helicopter strikes against rebel-held areas in Syria's north.

According to the London-based Syrian Observatory for Human Rights and video of the fighting posted on the Internet, the offensive at Taftanaz, an air base near the road that links the cities of Idlib and Aleppo, was being led by the Nusra Front, which the U.S. State Department designated a terrorist organization last month.

Ahrar al Sham, another rebel group that, like Nusra, has called for establishing an Islamic state in Syria, is participating in the offensive, according to its Internet postings.

The videos showed what appeared to be rebels in a commandeered armored vehicle driving near a fence on the base and firing at buildings in the distance, from which smoke could be seen rising, as well as fires raging near parked helicopters.

One activist said government forces, determined not to let the attackers seize the helicopters and warplanes parked on the tarmac, were seeking to destroy the aircraft themselves by shelling them.

The base is an important asset for the government, which has increasingly relied on helicopters to assault rebels and to resupply troops fighting to contain the nearly two-year uprising against Syrian President Bashar Assad.

Rebels have seized numerous air defense bases, such as the Marj al-Sultan military airport in Damascus, the capital, and raided them for weapons, but have faced a more difficult time retaining them when government forces counterattack.

The fighting at Taftanaz underscores the growing influence in the rebel movement of conservative Islamic groups such as Nusra, which has grown in the space of little more than a year from a small clandestine organization that largely carried out bombings into a group that fields battalions of fighters across the country.

The New York Times contributed to this report.