BEIRUT — Syrian rebels have begun a major operation in the Aleppo region, aiming to strike at security compounds and bases around Syria's largest city, activists said Friday.

It would be evidence that weeks of intense bombardments by the Syrian military, including airstrikes, have failed to dislodge the rebels. Instead, fighting rages across the country in a 17-month civil war that shows no sign of ending soon.

The rebel offensives in Aleppo are led by a brigade made up mostly of army defectors who specialize in operating artillery and tanks, said Mohammed Saeed, an activist based in the city.