"The secretary-general is shocked by the Israeli airstrikes in Gaza," UN spokesman Rolando Gomez told a press briefing in Geneva.

"He strongly appeals for the ceasefire to be respected, for unimpeded humanitarian assistance to be re-established, and for the remaining hostages to be released unconditionally."

Israel on Tuesday vowed to continue fighting in the Gaza Strip until the return of all hostages as it unleashed its most intense strikes since a truce took effect on January 19.

The health ministry in the Hamas-run territory reported at least 413 people killed in the latest strikes.

Hamas accused Israeli Prime Minister Benjamin Netanyahu of deciding to "resume war" after an impasse in truce negotiations, and warned that the return to fighting could be a "death sentence" for hostages still alive in Gaza.

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."

Brokered by Qatar, Egypt and the United States, the initial phase of the ceasefire took effect on January 19, largely halting 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.