Assad regime warplanes carried out at least 16 air strikes against the opposition in northern Hama province on Friday in the latest violence to interrupt a Russian and Turkish backed ceasefire, the Syrian Observatory for Human Rights said, after reports of clashes between regime forces and opposition fighters in an area outside Damascus.The British-based monitoring group did not immediately report any casualties. Fighting between government forces and rebels has also taken place near Damascus since the truce took effect, it said.

The Britain-based Syrian Observatory for Human Rights said it was not clear who had started the clashes in the Wadi Barada region, adding that regime helicopters were firing on the area.

"Clashes erupted and are continuing... in Wadi Barada near Damascus, with helicopters firing on positions belonging to the opposition and Fateh al-Sham Front," Observatory head Rami Abdel Rahman said.

Fateh al-Sham Front is the former Al-Qaeda affiliate previously known as Al-Nusra Front, which Assad regime said was excluded from the truce that began early Friday.

Syria's opposition however have said they understood the truce to apply to all of Syria, including areas where Fateh al-Sham is present.

Abdel Rahman said the clashes were a "violation of the truce, but it is not clear which party is responsible for starting it."

A resident of the area confirmed to AFP that he could hear heavy shelling on Friday morning.

Assad regime says the opposition in Wadi Barada, northwest of the capital, deliberately targeted water infrastructure that supplies the capital last week.

The damage in Wadi Barada and neighbouring Ain al-Fijeh has created severe water shortages in Damascus.

The UN said Thursday that four million people in Damascus and surrounding suburbs were now without water.

"People around Damascus have been without access to water in their homes for more than a week and are having to purchase water from private vendors, where prices and water quality are unregulated," the UN said.

The Observatory said the opposition targeted the water facilities in response to stepped up regime strikes on the area as the regime tries to impose a "reconciliation deal" on the region.

Over the past five months alone, such agreements have seen at least six towns around Damascus evacuated, with the opposition, their families and other civilians bussed to other opposition-held areas in northern Syria.

A nationwide truce brokered by Russia and Turkey came into force at midnight, and has largely held despite minor incidents elsewhere in the country.