The OSCE's Special Monitoring Mission in eastern Ukraine quoted local officials as saying a water pumping station and a section of pipeline near the town of Semyonovka, close to the bitterly contested city of Slavyansk, had been damaged in fighting between government forces and separatists.
"This pumping station and pipeline constitute the main water supply for Donetsk city's population of 1 million, and a further 3 million inhabitants of the region," it said in a statement following talks with Donetsk mayor Alexander Lukyanchenko.
The OSCE mission quoted the mayor on its Facebook page as saying that the water supply to Donetsk had not been affected yet, but that this was set to change "in a very short while".
Repair work was under way but help was needed to prevent it being interrupted by shooting, it said.
Semyonovka lies about 5 km (3 miles) southeast of Slavyansk and 110 km (70 miles) north of Donetsk, the main city in the Donbass coal mining region.
The statement is the latest sign of a growing humanitarian crisis in eastern Ukraine.
Water, food and electricity supplies have been affected in some areas and thousands of people have fled their homes for safe areas, including some who have taken refuge in Russia.
The separatists rose up in the Russian-speaking east in April to press for annexation by Moscow after Ukraine's pro-Russian president was toppled in February and Russia annexed the Crimea region from Ukraine in March.
Source: Google News