Air strikes have escalated in a rebel-held valley near Damascus containing the Syrian capital's main water supply, a day after insurgents and the government failed to agree a plan to repair the springs knocked out of service two weeks ago.
Several rebel groups warned that the escalation effectively meant the collapse of a shaky ceasefire brokered by Russia and Turkey.
The government and allied fighters from the Lebanese group Hezbollah launched an offensive two weeks ago to take back Wadi Barada, a valley where springs provided water to 4 million people in the capital.
The government says it wants to enter the valley to secure the water supply to the capital.
Rebels and local activists say pro-government forces are using the water issue to score a political victory weeks after the fall of Aleppo city.
"There is no longer any point in a ceasefire that is adhered to by one side. I think in the next few hours there will be an important development and a (decision) to freeze (enforcing) the ceasefire," said Mamoun Haj Musa, spokesman for the Free Syrian Army-affiliated Suqur al Sham rebel group, which is a signatory to the ceasefire.
The Syrian Observatory for Human Rights said at least 24 air strikes had hit the valley since Sunday morning.
A military media unit run by the Hezbollah said on Sunday it was suspending a ceasefire in the Wadi Barada area because rebels were disrupting negotiations and had opened fire on repair teams.
The Wadi Barada media office, run by local activists connected to the negotiating team, said it was untrue that any repair team had entered the valley, saying engineers had waited at the area's border while negotiations went on.
Rebels in Wadi Barada have allowed government engineers to maintain and operate the valley's pumping station, which supplies 70 per cent of Damascus and its surroundings, since they took control of the area in 2012.
The United Nations said the spring was damaged two weeks ago because "infrastructure was deliberately targeted", without saying who was responsible, and warned shortages in the capital could lead to waterborne disease outbreaks.
Rebels and activists say the spring was damaged by pro-government force bombardment. The government said rebels polluted the spring with diesel, forcing the state to cut supplies.
A U.N. spokesman said this week sabotaging civilian water supplies constituted a war crime.
