GENEVA — Syrian military airstrikes on rebels were responsible for severing water supplies to 5.5 million people in the Damascus region for weeks starting last December, the United Nations said on Tuesday, rebutting government claims that insurgents were to blame.

In a bombing campaign to drive rebel forces from the Barada Valley north of Damascus, Syrian air force jets launched multiple strikes on their positions around the al-Feijeh spring, which supplied water to the capital, the United Nations Commission of Inquiry monitoring the conflict in Syria said in a report.

The airstrikes amounted to a war crime, the commission said, because the effect of the attack — denying water to so many people — was “grossly disproportionate” to the military advantage that the government could have anticipated or achieved.

When water supplies to the capital were halted in late December, the government blamed rebels, first saying that they had poisoned the water and later that they had damaged the infrastructure. Water service was not restored until February.