But Mr. Guterres gave the inquiry a mandate only to look into attacks on humanitarian targets that had been supported by the United Nations or included in its “deconfliction” system, through which organizations could register their sites in the hope of protecting them from attack.

The inquiry looked at seven specific strikes carried out from April to July 2019 in opposition-held territory in northwestern Syria: on a school, a refugee camp, a children’s services center, three hospitals and a medical clinic. The board dropped one of the hospitals from its review, concluding that it did not match Mr. Guterres’s criteria.

It also determined that among the six attacks it had investigated, the Syrian government or its allies had committed all but the one against the refugee camp, which the board said was probably carried out by opposition forces.

“The charges in this report could not be more serious. And the incidents the report studied are the tip of the iceberg,” said David Miliband, president of the International Rescue Committee.

The Times has previously reported that Russian warplanes bombed a string of hospitals in northwestern Syria over one 12-hour period in May 2019 and then returned to bomb one of those hospitals again in November.

The hospital that was bombed twice, in the town of Kafr Nabl, was one of the sites under investigation by the board of inquiry. But instead of looking into the two Russian attacks, it focused instead on a separate attack on the same hospital, carried out by the Syrian government in July.

Before the inquiry’s publication of the report, Russia pressed Mr. Guterres not to release its conclusions, diplomats have said. Russia has vetoed 14 Security Council resolutions calling for action on Syria since 2011. In December, it blocked a resolution on cross-border aid deliveries from Turkey and Iraq to millions of Syrian civilians.