GENEVA — The Syrian Air Force first dropped barrel bombs from helicopters on a United Nations humanitarian aid convoy, then fired rockets from jets, then strafed survivors with machine guns, United Nations investigators said Wednesday in a report that found government forces had not only committed the attack in September but had done so deliberately, a war crime.
The attack, which killed 14 aid workers and stoked international outrage, was “meticulously planned” and “ruthlessly carried out,” the report said.
It called the attack “one of the most egregious” of many war crimes that investigators said had been committed during the Syrian government’s five-month offensive to take full control of the northern city of Aleppo.
The report, by a United Nations commission of inquiry that has been monitoring Syria’s conflict for years, is one of the most hard-hitting official assessments yet.
Its account of the convoy attack went much further than findings of a United Nations inquiry set up by Ban Ki-moon, then secretary general, which concluded in December that the convoy had been bombed from the air but did not identify the attackers.
The government of President Bashar al-Assad had no immediate comment on the new report, but it has repeatedly denied responsibility for the convoy assault or any other war crimes in the conflict, which Mr. Assad regards as a battle against terrorism.
Syrian officials and their Russian allies have suggested that insurgents were responsible for hitting the convoy, or even planes from the American-led coalition that has been bombing Islamic State targets in Syria. The Americans, who operate in areas far from the convoy assault site, have called such suggestions absurd.
The report released Wednesday found that war crimes had been committed by government and rebel forces, corroborating many of the worst allegations that Aleppo residents had made against both sides.
For months, the Syrian forces and their Russian allies bombarded eastern Aleppo as part of a strategy to force surrender, the commission’s chairman, Paulo Pinheiro, told reporters in Geneva, expressing frustration that the Syrian government had not cooperated with the investigation.
Mr. Pinheiro denounced what he called “the deliberate targeting of civilians” that killed many, including hundreds of children.
The Syrian government and Russia mainly used unguided, indiscriminate munitions on civilian areas, killing 300 people, including 96 children, in the first four days of a September offensive alone, the report said.
The report found that government aircraft repeatedly used chlorine bombs in Aleppo, violating a global treaty banning chemical weapons that Syria has signed. Just a day before the report was released, Russia and China vetoed a Security Council resolution to impose sanctions on some senior Syrian military officials and government bodies for earlier use of chemical weapons.
And the illegal tactics outlined in the report have appeared to continue throughout the week, further undermining the efforts in Geneva. Airstrikes have crippled hospitals in Idlib and the Damascus suburbs. Civilian areas in Douma and the Homs suburb of Al Waer have also been bombarded.
The strikes in Al Waer were said to have been in response to an attack by gunmen and suicide bombers on two security headquarters facilities in Homs, which killed 42 people. A hard-line Islamist group that is not a party to the talks in Geneva claimed responsibility.
Government aircraft, the commission said, carried out repeated attacks with barrel bombs laced with chlorine gas in the time covered by the report, from July 21 to Dec. 22, violating an international ban on chemical weapons.