The United Nations envoy for Syria has said government forces have dropped a large number of barrel bombs on Zabadani city, "causing unprecedented levels of destruction".
Staffan de Mistura cited local sources on Thursday, saying the air strikes caused many civilian deaths in the city, located about 45km northwest of the capital Damascus, and about 10km from the border with Lebanon.
"[The] Army of Fatah [a Sunni rebel alliance], that controls Zabadani, retaliated by firing rockets and mortar bombs on two Shia villages, Al Foua and Kefraya, near Idlib city in the north," de Mistura said.
"In both cases, civilians are tragically caught in the middle of the fighting."
Syrian state television said the army had destroyed a 70-metre-long tunnel used by the rebels to transport equipment into Zabadani.
At least 14 barrel bombs were dropped on several areas in Zabadani, the UK-based Syrian Observatory for Human Rights reported on Thursday - adding that a death toll could not yet be confirmed.
Fierce clashes continued during the bombardment in and around the city of Zabadani with reports of casualties on both sides, the observatory said.
Earlier this week, the Syrian army backed by the allied Lebanese Hezbollah movement advanced deeper into Zabadani, two weeks into a campaign to capture it from Sunni fighters, both sides said.
Sunni fighters say the army has now encircled them within a five square kilometre radius inside the city centre, and cut arms and food supplies from nearby towns.
But they had so far prevented the army and Hezbollah fighters from storming their defence lines in street fighting.
"They have advanced a bit but they have not entered inside the centre of city as they had expected they would in just a few days," said Abu Ado of the rebel group Ahrar al Sham.
Taking Zabadani from the rebels would be a strategic gain for the army, which is battling on several other fronts with a range of different groups.
Hezbollah's military role inside Syria has been growing steadily since the start of the conflict in 2011.
The government has described the group as its main ally in the fight against the rebels battling to topple Syria's President Bashar al-Assad.
Source: Al Jazeera And Reuters