Syria's main rebel groups say they have frozen their participation in preliminary peace talks planned for Kazakhstan later this month due to several "violations" by the regime of a five-day-old truce.
Sources told Al Jazeera the groups were freezing their participation in negotiations being prepared by Russia, due to a government-led assault on Wadi Barada, a rebel-held area near Damascus that is key to the capital's water supply.
The groups suspending their participation included the Free Syrian Army, one of the biggest coalitions fighting against President Bashar al-Assad, and the Army of Conquest, another coalition of anti-government groups.
"As these violations are continuing, the rebel factions announce ... the freezing of all discussion linked to the Astana negotiations," they said in a joint statement.
"The regime and its allies have not stopped shooting and have launched major and frequent violations, notably in the regions of Wadi Barada and Eastern Ghouta.
"Any [advance] on the ground goes against the [ceasefire] agreement and if things don't return to how they were before, the accord will be considered null and void," the statement added.
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Al Jazeera's Hashem Ahelbarra, reporting from the Turkish city of Gaziantep near the Syria border, called the FSA's announcement a "significant setback".
"The rebels say they signed the ceasefire in good faith but that the Syrian regime and its ally Russia have failed to live up to their end of the deal," he said.
"They say fighter jets have continued to pound rebel-held areas across the country with barrel bombs, particularly Wadi Barada."
For the last two weeks, Syria's air force, backed by Hezbollah and Shia militias, have launched almost daily bombing raids on Wadi Barada, some 15km from Damascus.
The Assad government is trying to seize control of the region, which supplies the main drinking water for roughly four million inhabitants of the capital and surrounding areas.
The UN's Office for the Coordination of Humanitarian Affairs (OCHA) said supplies had been cut since December 22 because "infrastructure was deliberately targeted and damaged", without saying who was responsible.
Water is now being rationed in Damascus as the government is relying on reserves.
The Syrian Observatory for Human Rights said about 1,000 civilians - all of them women and children - fled the fighting in Wadi Barada over the weekend, moving to other parts of the province.
Wadi Barada has been surrounded by government forces since mid-2015, but the siege was tightened in late December as the army piled on pressure seeking to secure a "reconciliation" deal.
It has won several of these deals in opposition areas around the capital, offering safe passage to surrendering rebels in return for retaking territory.
The opposition criticises them as a "starve or surrender" tactic.
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The violence threatens the delicate ceasefire, which has been in force since midnight Thursday, and is intended to pave the way for new peace talks in Astana later this month.
Turkey and Russia are organising the talks in Astana along with ally Iran, and say they are intended to supplement, not replace, UN-backed negotiations scheduled to resume in February.
Despite backing opposite sides in Syria's conflict, Ankara and Moscow have worked closely in recent months on the war, brokering a deal to evacuate civilians and surrendering rebels from Aleppo last month, before government forces recaptured the northern city in full.
The truce excludes the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group and Jabhat Fateh al-Sham, the group former known as the al-Nusra Front.
Source: Al Jazeera and news agencies