But neither side has been able to deliver a decisive blow, even though the Syrian military has superior weapons, including combat aircraft. In recent weeks, rebels have been targeting Syrian aircraft and air bases in hopes of reducing the regime’s advantage.

On Friday, amateur video posted by activists showed what appeared to be a Syrian government helicopter hurtling to the ground with a trail of white smoke behind it. Rami Abdul-Rahman, the head of the Observatory, said he was told by rebels that the helicopter was hit over Saqba, a town east of Damascus.

Another video, posted late Thursday, showed the purported capture of an air defense base by rebels. The video showed dozens of gunmen outside an area where smoke was billowing. Off camera, one gunman says a ‘‘missile air defense battalion’’ had been captured. Another video showed missiles inside a room.

Activist Mohammed Saeed, based in the Damascus suburb of Douma, said rebels captured an air defense base in the Eastern Ghouta area near the capital on Monday.

Syria imposes tight restrictions on foreign journalists and the content of amateur videos cannot be confirmed independently.

The rebels did not give any other evidence that would confirm the capture of a base, or identify the location of the video.

If confirmed, the capture of a stock of working anti-aircraft missiles would be a boost to a lightly-armed rebel force that says it faces frequent attacks by low-flying helicopters and warplanes.

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