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.
The battle for the town had been looming for some days, the BBC’s Jim Muir reports from Lebanon.
Zabadani is strategically placed near the main highway linking Damascus to Beirut and rebel control there has long been a thorn in the Syrian government’s side, our correspondent adds.
State TV quoted a Syrian army source as saying its forces were “making headway on several fronts” and the rebels had suffered heavy casualties.
Reports of fighting for Zabadani come after more clashes on Thursday and Friday in the northern city of Aleppo.
Syrian forces carried out air strikes in response to a rebel assault on several government-held areas. The state news agency Sana said the attack had been repelled and 100 rebels killed.
The rebels have driven government forces out of several areas in the countryside to the north of the city in recent months. They have also taken almost all of the neighbouring province of Idlib.
The uprising against President Bashar al-Assad began in March 2011 and since then more than 230,000 people are believed to have been killed.
Some 11.5 million others – more than half of the country’s population – have fled their homes.
The uprising includes Western-backed rebels and several Islamist fighting groups. Islamic State militants control large parts of north-eastern Syria and western Iraq.