The findings were contained in a new report by the Independent International Fact-Finding Mission on Iran, which examined US military conduct during the war on Iran and is due to be presented to the UN Human Rights Council in Geneva.
The report comes as US officials continue to defend their military aggression against Iran, claiming that American operations were conducted in accordance with the law of armed conflict.
The Pentagon rejected the findings.
“Our investigation remains ongoing, and we have nothing to announce at the moment,” it said in a statement.
White House spokeswoman Anna Kelly also attacked the UN Human Rights Council in response to the report, saying, “The UN Human Rights Council has accomplished nothing for human rights, spouting unserious nonsense for decades.”
The UN fact-finding missions are investigative rather than judicial bodies and therefore do not initiate criminal cases.
Their findings, however, can be used in criminal investigations by national courts or international tribunals such as the International Criminal Court.
States are due to discuss the findings when the report is presented to the Human Rights Council in Geneva on Monday.
The UN experts said they had found reasonable grounds to believe US forces were responsible for an attack on the Shajareh Tayyebeh Primary School in the city of Minab, where more than 168 people were killed, including around 120 schoolchildren, according to Iranian officials.
The mission concluded that the attack amounted to an indiscriminate attack that caused civilian deaths and damage to civilian infrastructure, constituting a war crime under international law.
The experts said the school was a clearly identifiable civilian object and found no evidence that it was being used for military purposes.
In a related finding, the experts reached a similar conclusion regarding a US strike on a sports Centre in Lamerd.
According to the report, the attack damaged nearby residential buildings and a school and killed and injured 22 civilians.
The mission concluded that the strike was indiscriminate and therefore constituted a war crime under international law.