The strikes, by far the deadliest since the truce came into force on January 19, killed more than 400 people, according to the health ministry in Hamas-run Gaza.

"Unfortunately, we are witnessing a situation in which we have an intolerable level of suffering for the Palestinian people, with the air raids that killed hundreds of people," Guterres told reporters at the UN in Geneva.

"We have humanitarian aid still blocked."

Guterres said it was the role of the United Nations to have the international community pressing for three "essential" points to be followed by Israel and Palestinian militant group Hamas.

"First, for the ceasefire to be fully respected. Second, for humanitarian aid to have access to Gaza in an unimpeded way. And third, for the unconditional release of hostages," said Guterres.

"And we will not give up on these objectives."

Israel said it had "no alternative" other than to resume military operations in order to bring home hostages.

- 'Hell on Earth' -

Philippe Lazzarini, head of the UN agency for Palestinian refugees, said resuming the war would fuel "hell on Earth".

"Awful scenes of civilians killed, among them children, following waves of heavy bombardment from Israeli forces overnight," the UNRWA chief said on X.

"Fuelling 'hell on Earth' by resuming the war will only bring more despair and suffering. A return to the ceasefire is a must."

UN rights chief Volker Turk said he was horrified by the strikes.

"This will add tragedy onto tragedy," he said in a statement.

"This nightmare must end immediately," Turk said, adding that "the last 18 months of violence have made abundantly clear that there is no military path out of this crisis".

"The only way forward is a political settlement, in line with international law. Israel's resort to yet more military force will only heap further misery upon a Palestinian population already suffering catastrophic conditions," he said.

"The hostages must be released immediately and unconditionally. All those arbitrarily detained must be released immediately and unconditionally. The war must end permanently," Turk said.

"We urge all parties with influence to do all in their power to achieve peace and avoid further suffering of civilians."

The initial phase of the ceasefire largely halted more than 15 months of fighting in Gaza triggered by Hamas's October 7, 2023 attack on Israel.

That first phase ended in early March, and the two sides have been unable to agree on the next steps.