Warplanes, tanks and artillery battered rebel-held parts of southern Damascus in what one Western diplomat said was a major escalation in Assad's campaign to crush rebels. Opposition activists said at least 10 people were killed there.
An air strike on Haram, a town in the northwestern province of Idlib near the Turkish border, killed at least 20 rebels of the Idlib Martyrs' Brigade, probably including their commander, Basil Eissa, the Syrian Observatory said.
Much of Idlib province is in the hands of insurgents, but remains vulnerable to air power, used increasingly by Mr. Assad's forces to contain his mostly Sunni Muslim opponents.
In Qatar, divided Syrian opposition groups were meeting to try to forge a cohesive leadership that would then make common cause with rebel factions fighting on the ground, in an effort to gain wider international recognition and arms supplies.
The Syrian National Council (SNC), the largest overseas-based opposition group, was expected to expand its membership to 400 from 300 and to elect a new leader and executive committee before talks with other anti-Assad factions in Doha this week.
Discussions focused on a proposal by influential opposition figure Riad Seif for a new structure combining the rebel Free Syrian Army, regional military councils and other insurgent units with local civilian bodies and prominent individuals.
Unity on Syria has also eluded major international powers since the conflict began in March 2011, with Russia and China opposing Western calls for his removal and critical of so far ill-coordinated outside efforts to arm his opponents.