On the first day of the war last Saturday, a strike hit an elementary school in the southern Iranian town of Minab and killed at least 150 people, say Iranian officials.

UN rights chief Volker Turk condemned "this absolutely tragic incident", and said he hoped investigations would be "prompt, and that they will be done in full transparency".

"We also expect accountability to be served, because obviously mistakes were clearly made," he told reporters in Geneva.

Neither Israel nor the United States has claimed responsibility for the attack, which was close to sites controlled by the Islamic Revolutionary Guards Corps (IRGC).

The US Department of Defense has said it is investigating the incident.

- US likely responsible -

The New York Times newspaper reported Thursday that US military statements indicating forces were attacking naval targets near the Strait of Hormuz, where an IRGC base is located, "suggest they were most likely to have carried out the strike".

An analysis of social media posts from the time of the attack, as well as photos and videos from witnesses, indicated that the Shajare Tayyebeh school had been struck at the same time as IRGC naval base sites, the Times said.

Two unidentified US officials told Reuters news agency that military investigators "believe it is likely" that US forces were responsible for the strike.

AFP has been unable to reach the location to independently verify the toll or the circumstances of the attack.

General Dan Caine, the top US military officer, said Wednesday that the United States had been carrying out strikes in southern Iran at the time.

The Times reported that he had presented a map indicating an area including Minab had been targeted by strikes in the first 100 hours of the operation.

Minab is around 25 kilometres (16 miles) inland from the strategically-important Strait of Hormuz.

Caine noted that Israel had mainly been operating further north in Iran.

- 'Horrible, tragic lesson' -

Turk on Friday welcomed the US announcement that it would investigate the incident.

"We need this to happen very quickly and we need to also make sure that there is accountability as well as redress for the victims," he insisted.

Turk, who said he hoped to go to Washington later this month, said there were "significant concerns about the respect for international humanitarian law, especially the conduct of hostilities... (and what) measures of precaution, of distinction, of proportionality are taken".

When it comes to a school, he said, that was "clearly a civilian institution that should never be attacked".

"Then there are questions around the type of weapons that were used, as well as the timing," he added.

He pointed out that the attack had "happened in the morning", at a time when children were likely to be in school.

"These factors need to be taken into account," he added.

The Norway-based rights group Hengaw said the school was holding its morning session at the time of the attack and reportedly had about 170 students present.

There is "a horrible, tragic lesson to be learnt when girls are killed in this way", Turk said.

He added that he hoped "there will be not only guarantees of non-recurrence but a review of all the standard operating procedures when it comes to this type of issues.

"The onus is now really on those who conducted these strikes to conduct this type of investigation," he said.

"We expect accountability to be served."