Further east, Syrian government troops lost ground to the Islamic State group near the key Deir Ezzor military airport.
The escalating violence augurs ill for peace negotiations in Kazakhstan later this month.
In the Wadi Barada region northwest of Damascus, fresh clashes broke out between rebel groups and advancing government forces, the Syrian Observatory for Human Rights said.
Regime shelling of the Deir Qanun village killed nine civilians including at least three children, the Britain-based monitor said in an updated toll.
Another 20 people were wounded in the attack.
Observatory head Rami Abdel Rahman said it was the "highest toll" there since the truce came into force on December 30.
Omar al-Shami, spokesman for the Wadi Barada Media Committee, gave a toll of 12 killed and said shells struck a temporary shelter where displaced women and children had been seeking refuge.
The activist group published gruesome pictures of what it said was the aftermath of the attack, including photographs of blood-stained floors and body parts wrapped in blankets.
Fighting has persisted for weeks in Wadi Barada, which is the main source of water for the capital.
Locals struck a truce with Syrian authorities on Friday to allow maintenance teams into Wadi Barada to restore water to Damascus, but the deal was called off after chief negotiator Ahmed al-Ghadban was killed Saturday.
Both sides accused each other of assassinating the retired army officer, who had only assumed his duties to restore the water supply that day.
"The agreement was considered null this morning after the regime's multiple violations and especially after the killing of the negotiations official," Shami said.
He told AFP the government's maintenance workers left Wadi Barada without completing the repairs, leaving 5.5 million people in Damascus and its suburbs without water.
An official from the reconciliation ministry earlier said the truce in Wadi Barada "had not completely collapsed".
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"We are communicating and pursuing new efforts in parallel with the ongoing military action," the official said.
Many battlefronts have quieted since the nationwide ceasefire brokered by regime backer Russia and opposition ally Turkey.
The deal excludes IS and its jihadist rival, the Fateh al-Sham Front -- formerly the Al-Qaeda-linked Al-Nusra Front.
On Sunday, IS fighters edged closer to Deir Ezzor military airport, despite heavy air strikes by government warplanes.
The Observatory said Sunday raids in Deir Ezzor and its outskirts killed nine civilians, including two women and two children.
"Clashes left 11 IS fighters dead and another three were killed in air strikes," Abdel Rahman said, without giving a toll for regime forces.
US invited to Astana
The second day of the jihadist group's brutal assault on regime-held territory around the city came despite more than 120 regime air strikes as well as heavy artillery fire, said Abdel Rahman.
A military source told AFP that Russian warplanes had carried out "a series of air strikes" around the airport and on several nearby hilltops.
IS unleashed a wave of tunnel bombs and suicide attacks against government forces on Saturday, leaving 12 government fighters dead.
Another 20 IS jihadists were killed in clashes.
The group has lost swathes of territory in northern Syria to Kurdish fighters as well as to a Turkish-backed rebel alliance, but it remains on the offensive in other parts of the country.
In addition to its push in Deir Ezzor, IS recently recaptured Palmyra in central Syria from government forces.
Syria's conflict began with protests against the rule of President Bashar al-Assad in 2011 but has since morphed into a multi-front war.
Although they support opposing sides in the war, Moscow and Ankara worked closely to negotiate the nationwide truce and are preparing talks in the Kazakh capital Astana set for January 23.
US president-elect Donald Trump's transition team has been invited to take part, but has not yet officially responded.