Syria's ceasefire appeared under threat as government forces intensified their attacks near Damascus and around 10 rebel groups said they were suspending talks about planned peace negotiations this month.
The talks are due to take place in the Kazakh capital Astana in late January but the rebels said they were pulling out of discussions due to "violations" by Damascus of a four-day old truce.
The talks are being organised by Russia, which supports the Syrian regime, and Turkey and Iran, which back the rebels.
"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 rebels said they "respected the ceasefire across the whole of Syria... but the regime and its allies have not stopped shooting and have launched major and frequent violations, notably in the (rebel) regions of Wadi Barada and Eastern Ghouta", near Damascus, they said.
For the past two weeks, even before the start of a nationwide truce brokered by Ankara and Moscow, Syria's air force has launched almost daily bombing raids on Wadi Barada, some 15 kilometres (10 miles) from Damascus.
"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 rebel statement added.
On Monday, the Syrian army backed by air strikes and artillery fire advanced as it battles to capture the area, which is key to the capital's water supply, the Syrian Observatory for Human Rights said.
"Regime forces and fighters from Lebanon's Hezbollah group are advancing in the region and are now on the outskirts of Ain al-Fijeh, the primary water source in the area," said Rami Abdel Rahman, director of the monitoring group.
He said government troops and allied fighters were engaged in fierce clashes with rebels, including former Al-Qaeda affiliate Fateh al-Sham Front, a claim denied by opposition fighters.
Two civilians were shot dead by snipers and two other civilians were killed in regime bombardment of the town of Rastan in central Homs province, the Observatory added.
Government forces have surrounded Wadi Barada since mid-2015, but the siege was tightened in December as the army piled on the pressure.
The Syrian government says rebels have targeted key water infrastructure, causing leaking fuel to poison water supplies and then cutting it off altogether.
The United Nations says at least four million people in Damascus have been without water since December 22.
The Observatory said around 1,000 women and children fled the area over the weekend.
The ceasefire deal, and the plan for new talks, received the unanimous backing of the UN Security Council on Saturday, despite offering a competing track to UN-sponsored negotiations.
Turkey and Russia say the talks 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 the regime recaptured the northern city in full.
Both countries are also waging their own military interventions in Syria, with Russian forces fighting to bolster President Bashar al-Assad's government since September 2015.
Turkey launched a military campaign in northern Syria in August 2016, targeting the Islamic State group but also Kurdish militants.
The Syrian conflict has also spilled over into Turkey, with several attacks blamed on Kurdish or IS.
IS claimed responsibility on Monday for a New Year's Eve attack on an Istanbul nightclub that killed 39 people.
Turkish Deputy Prime Minister Numan Kurtulmus warned that Ankara was determined to press on with its military operation in Syria "until these terror organisations no longer remain a threat to Turkey".
A string of efforts to find a political solution to Syria's war have failed since it began with anti-government protests in March 2011.
The conflict has killed more than 310,000 people, and displaced over half the population, including millions who have fled abroad, becoming refugees.