According to Lebanese group Hezbollah's television station, the Syrian army and its allied militia launched a major assault on the rebel-held Syrian city of Zabadani on Saturday.
It said heavy artillery and aerial bombardment were being deployed to capture Zabadani, located north-west of the Syrian capital near the frontier with Lebanon.
Footage released on the channel showed large plumes of fire rising from the city. The Syrian army, with its Shi'ite ally Hezbollah, has long sought to wrest control of Zabadani from Sunni militants.
The city is close to the Beirut-Damascus highway that links the two countries and capturing it would be a major strategic gain for Syrian President Bashar al Assad's government.
The former popular resort city is one of the rebels' last strongholds along the border. It was part of a major supply route for weapons sent by Syria to Hezbollah before the outbreak of the Syrian conflict in 2011. Violence from the four-year civil war has regularly spilled over into Lebanon.
The Syrian military and pro-government fighters have regularly clashed with insurgents in the mountainous area north of the capital. The rebel groups in the area include al Qaeda's Syrian wing, the Nusra Front.
Iranian-backed Hezbollah has stepped up its assault on rebel outposts along the Qalamoun mountain region straddling the Lebanese Syrian border in recent months.
An announcement of the start of a major military campaign by the Syrian army and the Lebanese group to capture Zabadani had been expected in recent days.
The rebels say they have planted mines around the city, which is mostly deserted, and are well prepared to repel the assault. The Syrian army is fighting on several other fronts; as well as battling rebels around the southern city of Deraa and the northern city of Aleppo, it has been fighting Islamic State as the militant group attempts to seize government-held areas of the northeastern city of Hasaka.