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The Observatory said the rebels attacking Taftanaz base included members of Jabhat al-Nusra, affiliated with al Qaeda and branded a terrorist organization by the United States. The group’s fighters have been among the most effective on the rebel side in their battle to oust Mr. 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. 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 on Wednesday bombarded the Mannagh air base, which has been subjected to almost daily attacks since late last month.

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, which is mostly in the hands of rebels.

Anti-regime activists say more than 45,000 people have been killed since the uprising against Mr. 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.