They also accuse Iran's theocratic government of carrying out crimes against humanity against its own people in a report published on Thursday.
The report adds to international evidence gathered about crimes in the war in Iran, and could eventually be used in efforts to reach justice in international courts.
The fact-finding mission on Iran, chaired by jurist Sara Hossain of Bangladesh, is made up of independent experts and was created in 2022 by the Human Rights Council, which is based in Geneva.
The fact-finding mission says its findings on US war crimes centre on a pair of US air strikes that killed at least 178 civilians, including women and children.
At the start of the US-Israeli war against Iran in February, at least one US missile hit the Shajareh Tayyebeh school in Minab, according to an Associated Press investigation.
There has been no final accounting of what happened.
Iranian state media have said 168 people were killed in the strike, most of them children.
"Following the United States and Israeli attacks on 28 February, the mission found reasonable grounds to believe the United States committed the war crime of launching indiscriminate attacks resulting in the loss of life or injury to civilians or damage to civilian objects," a UN statement about the team's findings said.
The US State Department did not immediately respond to a request for comment.
In their report, the experts concluded that "the building of the school was the intended point of impact" and the resulting damage was not the result of an errant strike or collateral damage from an attack on a compound of Iran's Revolutionary Guard next to the school.
The UN-backed team based its allegations on two strikes - one involving Tomahawk missiles that hit the "clearly identifiable" school, and another in which Precision Strike Missiles dispersed tungsten pellets over a "clearly identifiable sports complex and residential area" in the southern city of Lamerd, also on February 28.
The UN-backed team said 22 civilian men and women were killed.
US Central Command has denied responsibility for the strike on Lamerd.
The team found that killings by state security forces in Iran have occurred on a "staggering scale", and civilians faced "gross human rights violations and crimes against humanity committed by their own government".
Iranian security forces carried out "violent repression" in the past two years by firing assault rifles into crowds, beating protesters and arresting healthcare workers, among other things, the team said.
Detainees faced torture, and were deprived of legal counsel, leaving their families "in the dark" about their whereabouts.
The experts said the United States and Iran should halt all rights violations and crimes, and provide "full reparations to victims".
