The Syrian capital Damascus has seen some of the heaviest fighting of the conflict so far, according to reports from activists and residents.
Plumes of smoke were seen rising from the suburbs as the army sought to drive out the Free Syrian Army.
The fighting comes after the International Committee of the Red Cross said the conflict in Syria was now in effect a civil war.
It means combatants across Syria are now subject to the Geneva Conventions.
It had previously regarded only the areas around Idlib, Homs and Hama as warzones.