GENEVA- The U.N. human rights office on Tuesday urged what it called the forces behind a deadly attack on a girls' school in Iran to investigate and share insights into the incident, without saying who it believed was responsible.

"The High Commissioner (Volker Turk) calls for a prompt, impartial and thorough investigation into the circumstances of the attack. The onus is on the forces that carried out the attack to investigate it," U.N. human rights office spokesperson Ravina Shamdasani told a Geneva press briefing.

"This is absolutely horrific," Shamdasani said, adding that images circulating on social media captured "the essence of the destruction, despair and senselessness and cruelty of this conflict".

Turk also urged all parties to exercise restraint and to return to the negotiating table, she said.

The school in southern Iran was hit on Saturday, the first day of U.S. and Israeli attacks against the country. Secretary of State Marco Rubio said on Monday that U.S. forces "would not deliberately target a school". Israel has said it is investigating the incident.

Iran's ambassador to the U.N. in Geneva Ali Bahreini had previously raised the issue with Turk in a letter dated March 1, calling the attack "unjustifiable" and "criminal".

He said the attack had killed 150 students.

Turk's office does not have enough information to make a determination as to whether the strike constituted a war crime, Shamdasani said.

-Reporting by Emma Farge; Editing by Ludwig Burger, Linda Pasquini and Aidan Lewis/Reuters