UN human rights experts say the Syrian air force deliberately bombed a spring outside Damascus in December, cutting off the water supply for 5.5 million people living in and around the city.
The Syrian government blamed rebels for damaging the Ain al-Fijeh spring during the battle for the Wadi Barada valley.
But a new report by a UN commission of enquiry says evidence showed the damage was caused by at least two air strikes.
The attack, it concludes, was grossly disproportionate and was a war crime.
The report also says the Syrian air force committed another war crime by bombing a complex of schools in rebel-held Idlib province in October - something for which the government also denied responsibility.
Rebel and jihadist groups are meanwhile accused of carrying out indiscriminate attacks with indirect artillery fire, resulting in dozens of civilian casualties.
The report was published a day before the sixth anniversary of the start of the uprising against President Bashar al-Assad, which evolved into a civil war that activists say has left more than 320,000 people dead.