It also found Iranian authorities committed crimes against humanity during their deadly crackdown on anti-government protests.
The findings on Thursday in a new report by the Independent International Fact-Finding Mission on Iran, to be submitted to the UN Human Rights Council in Geneva, examined US conduct during the Iran war that began in February and the Iranian government's response to nationwide unrest that began in late December.
The Pentagon declined to comment on a report conducted by an outside organisation.
The Iranian permanent mission in Geneva did not immediately respond to Reuters requests for comment.
US officials have previously said military operations are conducted in accordance with the law of armed conflict, while Iran has consistently rejected allegations of systematic rights violations.
UN Fact-Finding missions are investigative, not judicial bodies, and therefore do not initiate any criminal cases. Their findings, however, can be used in criminal investigations by national courts or international tribunals such as the International Criminal Court. States will discuss the findings of the report when it is presented to the Human Rights Council in Geneva on Monday.
The UN experts said they 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 150 people were killed, including around 120 schoolchildren, according to Iranian officials.
The attack constituted an indiscriminate attack causing civilian deaths and damage to civilian infrastructure, amounting to a war crime under international law, the mission concluded.
According to the report, the US failure to update information in relation to the school, which was adjacent to an Islamic Revolutionary Guard Corps naval complex, in its targeting databases and to confirm the building was a military objective amounted to more than mere negligence.
"Rather, the US directed the strikes at the building of the school while being aware of a substantial risk of striking a civilian object and acting recklessly as regards the possibility that this would happen," the report read.
The UN mission probe found the Iranian authorities carried out a widespread and systematic attack against civilians during protests that erupted in late December, involving unlawful killings, torture, arbitrary detention, enforced disappearances and severe restrictions on freedom of expression.
Rights groups say bystanders were among those killed during the biggest crackdown since Shi'ite Muslim clerics took power in the 1979 revolution. Tehran blamed "terrorists and rioters" backed by exiled opponents and foreign foes the US and Israel.
The probe said it could not independently verify the death toll, but that it believed the number of dead and injured was likely far higher than official figures which say 3,038 people were killed, and 25,000 people injured.
The government's response to the protests — including violence and killings, cutting off the internet and use of the death penalty — marked a significant escalation from previous patterns of suppressing dissent, the report said.
