The water from rebel-held Wadi Barada, which supplies four million people in Damascus, has been cut since December 22, causing major shortages.
The regime and rebels have traded accusations over responsibility.
Government forces backed by Lebanon's Hezbollah group are fighting to recapture the area northwest of Damascus even as a nationwide ceasefire has brought quiet to other parts of Syria in preparation for renewed peace talks.
The truce, brokered by regime backer Russia and rebel sponsor Turkey, was meant to pave the way for peace negotiations later this month in the Kazakh capital Astana.
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The Observatory, a Britain-based monitor of Syria's conflict, said pro-regime forces and rebels were locked in ongoing clashes there Thursday.
Retaking the area became an urgent priority for the regime as the cut caused severe water shortages in the capital.
Damascus residents said they have been forced to buy bottled water at twice its normal price as their supplies have run low.
"We used to complain about power cuts, but now we can see that it's nothing compared to the lack of drinkable water. Water is life," said Faiz, a 50-year-old civil servant.
The public water authority has published daily updates on its Facebook page telling residents where and when water will be distributed.
But locals said the water often fails to arrive and when it does, is undrinkable.
Authorities said they closed several shops on Wednesday for selling drinking water at elevated prices.
Mohannad, 53, said he buys drinking water at twice its normal price. A 10-litre bottle that sold for 500 liras ($1/95 euro cents) before the mains water was cut now sells for 1,000 liras.
The regime says forces in Wadi Barada include former Al-Qaeda affiliate Fateh al-Sham Front, previously known as Al-Nusra Front, which Moscow and Damascus say is excluded from the ceasefire.
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