Israel is facing international criticism over its deadly airstrike on a school sheltering displaced citizens in northern Gaza.

Officials in Gaza say the attack on Saturday killed more than 100 people and wounded dozens of others.

Israel has repeatedly struck schools in the enclave, claiming they serve as bases for the Islamic group Hamas.

The European Union's foreign policy chief Josep Borrell wrote on social media that "at least 10 schools were targeted in the last weeks" by Israel. He added, "There's no justification for these massacres."

British Foreign Secretary David Lammy said in his social media post that he is "appalled" by the Israeli military strike on the school and the "tragic loss of life."

He also wrote: "Hamas must stop endangering civilians. Israel must comply with International Humanitarian Law."

Tensions are running high in the Middle East with Iran vowing to take revenge on Israel over the killing of the political leader of Hamas while he was visiting Tehran late last month.

Countries trying to mediate a ceasefire between Israel and Hamas are calling for the resumption of negotiations on August 15.

According to Iran's state-run news agency, the country's UN mission said on Friday that Iran's priority is to establish a lasting ceasefire in Gaza.

But it added that Iran has "the legitimate right to self-defense," and that this is "a matter totally unrelated" to a Gaza ceasefire.

The Iranian mission reportedly said, "We hope that our response will be timed and conducted in a manner not to the detriment of the potential ceasefire."

The comments suggest that Iran is carefully studying the timing and method of retaliation, while weighing the possible impact on the ceasefire talks.