Fierce fighting is reported in Syria's capital Damascus and the city of Aleppo as government forces seek to regain control of rebel-held areas.
The Syrian army is reported to have launched an assault on a district of Damascus, the BBC says.
Activists and local residents say helicopters are attacking targets on the ground, as tanks and armoured personnel carriers enter the streets.
Troops are also said to have deployed in the western suburb of Mezzeh.
Fighting continued for a third day in the city of Aleppo, where activists say a building collapsed under tank fire.
The violence follows a week in which rebels made major advances, taking control of several parts of Damascus, seizing border crossings and claiming an attack that killed four top security officials, including the defence minister .