BEIRUT: The United Nations envoy for Syria has voiced deep concern about civilians in the city of Zabadani, the focus of an offensive by the Syrian military and Lebanon's Hezbollah aimed at wresting control of the area from insurgents.

Citing local sources in a statement late on Tuesday, Staffan de Mistura said the military had dropped a large number of barrel bombs on Zabadani "causing unprecedented levels of destruction and many deaths among the civilian population."

Control of the city, about 45 km (30 miles) northwest of the capital Damascus, is seen as crucial to consolidating President Bashar al-Assad's control over the border zone between Lebanon and Syria.

The Syrian air force has bombarded areas in and around the city and Sunni insurgents have retaliated by firing rockets and heavy mortar shells on two villages near Idlib city in the north, de Mistura said.

An alliance of insurgents known as the "Army of Fatah" (Islamic Conquest) had targeted Al Foua and Kefraya, northern villages where a large number of civilians are trapped, he said.

"In both cases, civilians are tragically caught in the middle of the fighting," he added. Al Foua and Kefraya are home to Shi'ite populations.

Earlier this week the Syrian army backed by Hezbollah advanced deeper into Zabadani, two weeks into a campaign to capture it from insurgents, rebels and the army said.

Taking Zabadani from the insurgents would be a strategic gain for the Syrian army which is battling on several other fronts with a range of different insurgent groups.

Hezbollah's military role inside Syria has been growing steadily since the start of the conflict in 2011. The Syrian government has described the group as its main ally in the fight against the insurgents battling to topple Assad.