Speaking at the start of an urgent debate in the UN Human Rights Council focused on the February 28 strike on an Iranian elementary school in Minab on the first day of the Middle East war, Volker Turk said that "whatever differences countries have, we can all agree they will not be solved by killing schoolchildren".

The bombing "evoked a visceral horror", Turk told the United Nations' top rights body.

"The images of bombed-out classrooms and grieving parents showed clearly who pays the highest price for war: civilians with no power in the decisions that led to conflict," he said.

"In this case, a reported 168 pupils, teachers, school staff, and their loved ones."

A US Tomahawk cruise missile hit the school due to a targeting mistake, according to the preliminary findings of a US military investigation reported by The New York Times newspaper.

The UN rights chief stressed that "the onus is on those who carried out the attack to investigate it promptly, impartially, transparently and thoroughly".

"Senior US officials have said the strike is under investigation," he said, calling "for that process to be concluded as soon as possible, and for its findings to be made public".

"There must be justice for the terrible harm done."

Friday's discussion, at the request of Iran, China and Cuba, marked the second urgent debate before the rights council this week, focused on the war in the Middle East.

On Wednesday, a debate requested by Bahrain on behalf of the Gulf Cooperation Council and Jordan, considered Iran's strikes on countries across the Gulf region and their impact on civilians.

Following that debate, the 47-member council approved by consensus a resolution condemning Iran's "egregious attacks" on its Gulf neighbours, calling for swift "reparation" to all victims of its strikes.

The council was not asked to consider a draft resolution during Friday's debate, which was scheduled to last around two hours.