BEIRUT (AP) -- Rebels attacked a sprawling air base in northern Syria on Wednesday as the opposition expanded its offensive on military airports in an attempt to sideline a major weapon in the hands of President Bashar Assad's forces.
The Britain-based Syrian Observatory for Human Rights said the rebel assault on the Taftanaz base was preceded by heavy shelling of the area, and the fighters appeared to be trying to storm the facility. Observatory director Rami Abdul-Rahman described the attack as one "of the most intense" on the airfield.
In the past few weeks, Syrian rebels have stepped up their attacks on airports around the neighboring province of Aleppo, trying to chip away at the government's air power, which poses the biggest obstacle to the opposition fighters' advances. As its control of large swaths of territory has slipped over the past year, the government has increasingly relied on its warplanes and helicopters to strike rebel-held areas.
Several past rebel attempts to capture the Taftanaz base have failed.
The Observatory said Syrian army helicopters were helping defend the airfield against the rebel assault. It added that four rebels were killed in the clashes around the base and that one helicopter was hit by rebel fire.
The Observatory said the rebels attacking Taftanaz base included members of Jabhat al-Nusra, which has been branded a terrorist organization by the U.S. and is affiliated with al-Qaida. The group's fighters have been among the most effective on the rebel side in their battle to oust Assad.
On Tuesday, clashes between government troops and rebels forced the international airport in Aleppo to stop all flights in and out of Syria's largest city.
Rebels have been fighting for control of Aleppo since launching an offensive on the city over the summer. The fight over the commercial hub has turned into a bloody stalemate, although rebels have captured large swathes of territory in the surrounding Aleppo province west and north of the city up to the Turkish border.
The rebels have been attacking three other airports in the Aleppo area, including the Mannagh military helicopter base near the Turkish border. They have posted dozens of videos online that appear to show fighters shooting mortars, homemade rockets and sniper rifles at targets inside the bases.
The Local Coordination Committees, another activist group, said rebels Wednesday bombarded the Mannagh air base, which has been subjected to almost daily attacks since late last month.
The Observatory and the LCC said Syrian army warplanes bombed several Damascus suburbs including Maadamiyeh, where they said there were casualties.
The Observatory also reported that eight shells struck the Palestinian refugee camp of Yarmouk in the capital forcing some of the residents to flee the area that is mostly in the hands of rebels.
Anti-regime activists say more than 45,000 people have been killed since the uprising against Assad began in March 2011. Since then, it has evolved into a full-scale civil war with scores of armed groups across the country fighting regime forces.