The Syrian army and Lebanese group Hezbollah said they had launched a major ground and air assault on the rebel-held Syrian city of Zabadani on Saturday and were closing in on the insurgents holed up inside.

The army, with its Shiite ally Hezbollah, has long sought to wrest control of Zabadani from the Sunni rebels. The city is near the Lebanese border, and the Beirut-Damascus highway that links the countries, and capturing it would be a major strategic gain for Syrian President Bashar al Assad's government.

Footage released on Hezbollah's TV channel al Manar and Syrian state TV showed large plumes of fire rising from the city, and the sounds of aerial bombardment and heavy artillery shelling could be heard.

The once popular resort city, north-west of the capital Damascus, 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 2011 outbreak of the Syrian conflict, which has killed over 200,000 people.

The Syrian army said it had inflicted heavy casualties on "the terrorist groups fortified inside the city" and was advancing from several fronts towards their positions.

A hilltop west of Zabadani that overlooks rebel positions, known as Qalat al Tel, was also captured, the army said.

The rebels said they had planted mines around the city, which is now mostly deserted, and were well prepared to repel the assault.

The Syrian military and pro-government fighters have regularly clashed with insurgents in the mountainous area north of the capital, and violence from the four-year-old civil war has regularly spilled over into Lebanon. The rebel groups in the area include al Qaeda's Syrian wing, the Nusra Front.

Iranian-backed Hezbollah - which has been a crucial ally to Assad in the war, sending fighters to bolster his forces - has in recent months stepped up its assault on rebel outposts along the Qalamoun mountain region straddling the Lebanese Syrian border.

The aim of the campaign was to cut supply arms routes of rebels along the porous rugged border terrain.

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 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.

(Editing by Pravin Char)