The UN children's agency said that the fierce fighting that has rocked Aleppo in recent weeks had damaged the electricity networks needed to pump water supplies throughout the divided city.
"Children and families in Aleppo are facing a catastrophic situation," Hanaa Singer, UNICEF's representative in Syria, said in a statement.
"These cuts are coming amid a heatwave, putting children at a grave risk of waterborne diseases," she said, stressing that "getting clean water running again cannot wait for the fighting to stop. Children's lives are in serious danger."
Temperatures in Aleppo were pushing 40 degrees Celsius (104 degrees Fahrenheit) on Tuesday, according to BBC Weather.
UNICEF pointed out that attacks on July 31 had struck the electricity transmission station which powered water pumping to the eastern and western parts of the city.
Authorities had urgently restored an alternative power line on August 4, only to see it damaged less than 24 hours later, with intense fighting hampering repair efforts.
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"As a result, the whole city has been without running water for four days," the UNICEF statement said.
The UN, which has called for an urgent "humanitarian pause" in the fighting in Aleppo to help trapped civilians, estimates that some two million people are in danger of coming under total siege.
That number includes up to 275,000 people believed to be trapped in rebel-controlled east Aleppo, and well over one million who are in the government-controlled western part of the city, as well as those in the suburbs.
Experts with the Syrian Observatory for Human Rights meanwhile put the embattled city's population at no more than 1.5 million.
UNICEF said it and its partners were scaling up their emergency response to bring safe drinking water to civilians in Damascus.
But it warned that civilians would soon be forced to resort to unsafe water sources if the pumping systems were not quickly repaired.
"We urge parties to the conflict to immediately allow safe access for technicians to conduct critical repairs to the electricity and water systems," Singer said.
More than 290,000 people have been killed since Syria's conflict erupted in March 2011.