Hundreds of Islamic militants fighting to topple Syrian President Bashar Assad have seized parts of a strategic north-western air base after weeks of battling government troops for control of the sprawling facility, Belfast Telegraph reported.

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 the Taftanaz air base in the northern Idlib province and had seized control of more than half of it. Intense battles were still raging. One activist said rebels had suffered losses.

An activist near Taftanaz said the government bombed the air base from warplanes in a desperate attempt to push back rebels who seized several helicopters. The account could not immediately be confirmed.

An amateur video posted by activists online showed smoke rising from behind helicopters parked on the Taftanaz tarmac, and a narrator said it was the result of an air strike.

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.