Syrian forces with their Lebanese Hezbollah allies are carrying out a major offensive against rebels holding the strategically important town of Zabadani, state TV says.
Zabadani, north-west of the capital, Damascus, is the last significant town held by Sunni Muslim rebels in an area close to the Lebanese border.
Hezbollah's al-Manar TV showed large clouds of smoke from explosions caused by artillery and air strikes.
Reports say civilians have fled.
The rebels remaining in the town are thought to have been reinforced by hundreds of fighters who fled hills on the nearby Lebanese border. They were ejected from the hills by Hezbollah, a Shia Muslim force.
Attempts to negotiate a peaceful outcome to the conflict in Zabadani failed, al-Manar reported.