Jens Laerke, a spokesman for the United Nations humanitarian office in Geneva, said by email Tuesday that the “deliberate targeting of the water infrastructure” had caused the shut-off.

“But we are not in a position to say by whom,” he said. “The area has been the scene of much fighting, so we have not been able to access it.”

Now, 5.5 million people in Damascus and the vicinity lack water, which has raised the risk of waterborne disease, especially among children, he said.

Fighting near the Barada Valley has continued despite the cease-fire.

Antigovernment activists say that government forces, and fighters from Lebanon’s Hezbollah organization, have continued to attack the area in an apparent attempt to take it over. The Syrian Observatory for Human Rights, which tracks the conflict from Britain through a network of contacts in Syria, said the government launched 15 airstrikes on the area Monday amid clashes between rebels and pro-government forces.

Turkey’s foreign minister, Mevlut Cavusoglu, accused the Syrian government and its allies on Wednesday of violating the cease-fire, saying that the new violence could derail peace talks meant to be held in Astana, the capital of Kazakhstan, on Jan. 23. Rebel groups have threatened to boycott the talks if government attacks do not stop.

Few Damascus residents expect much from the talks or have time to think about them. While generally safe from the violence that had reduced other parts of the country to rubble, they were struggling through a cold winter of high prices and scarce commodities before the water crisis, making things worse.

The Syrian government has sought to ease the crisis by trucking water from wells around the city, and the United Nations has rehabilitated 120 wells to cover about one-third of the city’s daily needs, Mr. Laerke, the spokesman, said.