The Syrian government intentionally bombed the Ain al-Fijeh spring in December, leaving more than five million people in Damascus without access to water, a UN probe said Tuesday, as it branded the strike a "war crime".
"The information examined by the Commission confirms that the bombing of (the Ain al-Fijeh) spring was carried out by the Syrian Air Force," the UN Commission of Inquiry on Syria said in a report.
The report meanwhile dismissed regime allegations that rebels had contaminated the water.
Around 5.5 million people in Damascus and its suburbs were cut off from water when fighting intensified in Wadi Barada near Damascus in late December.
The regime accused the rebels of poisoning water resources and cutting off the mains, while the armed opposition said regime bombardment had destroyed the infrastructure.
The UN experts, who have never been granted access to Syria and who base their reports on interviews and documents, said they had found no "indications that the water was contaminated" before the spring was bombed on December 23.
"On the contrary, interviewees say that Wadi Barada residents used water up until the bombing of 23 December and no one experienced any symptoms of contamination," the report said.
Following the bombing, the water was contaminated after shrapnel damaged fuel and chlorine storage facilities, it said.
The bombing itself indicated that the "spring was purposely targeted," the report found.
"While the presence of armed group fighters at (the Ain al-Fijeh) spring constituted a military target, ... the damage caused ... was grossly disproportionate to the military advantage anticipated or achieved," it said.
"The attack amounts to the war crime of attacking objects indispensable for the survival of the civilian population, and further violated the principle of proportionality in attacks," the report concluded.
At the end of January, Syria's army regained control of Wadi Barada, which rebels first seized in 2012.
More than 320,000 people have been killed and millions forced to flee their homes since Syria's conflict erupted in March 2011, as protests against President Bashar al-Assad morphed into war following a government crackdown.