At least 153 people including children have died after a reported strike hit a school in southern Iran, according to Iranian officials.
Iran has blamed the US and Israel. The US military's Central Command (Centcom) said it was looking into reports of the incident, while Israel's military said it was "not aware" of any IDF operations in the area.
The girls' school was located in Minab, near an Islamic Revolutionary Guard Corps (IRGC) base which has previously been a target.
At least 201 people have been killed in air strikes in Iran and 747 hurt since Saturday, the Iranian Red Crescent said.
Iran's President Masoud Pezeshkian called the incident a "barbaric act" and "another black page in the record of countless crimes committed by the aggressors".
In a statement reported in US media, Centcom spokesman Tim Hawkins said: "We take these reports seriously.
"The protection of civilians is of utmost importance, and we will continue to take all precautions available to minimise the risk of unintended harm."
The six-day working week in Iran runs from Saturday to Thursday, with Friday being the only official day of rest, meaning the school was likely occupied when it was hit.
Following the incident on Saturday, Red Cross and Red Crescent officials in Geneva said the society had mobilised response teams to go to the school.
An official said that the school, in the town of Minab in Hormozgan province, had been "targeted by three missile attacks".
It is located about 600m (1970ft) from the base of the IRGC.
The BBC has verified clips of the aftermath of the explosion, which show smoke rising from a building as crowds gather nearby and people can be heard screaming in panic.
But it has not been able to independently verify the death toll - international news organisations are often refused visas to Iran which severely limits their ability to gather information there.
Iranian social media users have reacted with anger to news of the incident.
An Iranian living abroad who opposes military intervention in Iran commented: "The first victims of this war are 40 girls in Minab, hit by a missile attack. Is this the war you cheer for?"
Deep mistrust of the Iranian regime, however, makes official reports difficult for many to accept, and some Iranians directly blamed the regime for the attack.
One user wrote: "Even if the regime did not directly target schools, the deaths of children in Minab remain the responsibility of the Islamic Republic.
"People have no shelters, the internet is cut, phone lines are down, and there has been no warning to keep children out of school. In these conditions, the minimum requirement should be to stay at home."
The school tragedy took place as the US and Israel launched wave after wave of air strikes against targets in a number of Iranian cities throughout Saturday.
The attacks continued on Sunday, following the killing in an air strike of Iran's Supreme Leader Ali Khamenei, together with many of his senior commanders.
