Clashes erupted between Syrian government forces and opposition fighters in an area outside Damascus on Friday, despite a nationwide truce that began at midnight, a monitor said.
The Britain-based Syrian Observatory for Human Rights said it was not clear who had started the clashes in the Wadi Barada region.
"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 Syria's government 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.
Syria's government says rebels 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 rebels targeted the water facilities in response to stepped up regime strikes on the area as the government 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 rebels, their families and other civilians bused 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.
The Observatory said Friday afternoon that Syrian warplanes had carried out at least 16 strikes in the north of Hama province, in central Syria.
The monitor said there were no deaths reported in the strikes, which hit areas controlled by a variety of opposition forces, including groups that have signed on to the ceasefire deal.
