Homs, Syria's third largest city and a center of the uprising against Assad, was subjected to intense regime shelling in February and March, the first area to suffer widespread devastation. The regime assault slowed in April, as the focus shifted to other areas, including Aleppo, where rebels first seized control of some areas in an offensive in late July.
After weeks of stalemate in Aleppo, rebel fighters announced a new push last week to take the city, but the regime has fought back hard, shelling from tanks and bombing from the air.
An AP journalist in Aleppo said fighting has intensified this week, and that the rebels appear to be losing some ground in close-quarter combat. Over a period of a few days, rebels lost control of several buildings in one of the front-line neighborhoods, Saif al-Dawla, he said.
Syria's conflict began with a peaceful uprising against Assad, inspired by last year's Arab Spring rebellions against authoritarian rulers. Amid an escalating regime crackdown, the rebellion gradually turned into a civil war. The regime's troops are stretched thin, enabling rebels to control large stretches of countryside in Syria's most densely populated west.
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.
Associated Press Writer Suzan Fraser In Ankara, Turkey, contributed to this report.
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