The strikes, by far the largest since a truce took effect in January, killed more than 400 people across the Gaza Strip, according to the health ministry in the Hamas-run territory.

By Tuesday afternoon, witnesses said the attacks had largely stopped, though sporadic bombing continued.

"Today I felt that Gaza is a real hell," said Jihan Nahhal, a 43-year-old woman from Gaza City, adding that some of her relatives were wounded or killed in the strikes.

"Suddenly there were huge explosions, as if it were the first day of the war."

Israel vowed to keep fighting until the return of all the hostages seized by Palestinian militants during the October 2023 attack that sparked the war.

Hamas, which has not responded militarily so far, accused Israel of attempting to force it to "surrender".

Foreign Minister Gideon Saar said that "without the release of our hostages, Israel has no alternative but resuming military operations".

He later said that the strikes were "not a one-day attack", adding Israel would "continue the military operation in the coming days".

The White House said Israel consulted US President Donald Trump's administration before launching the strikes, while Israel said the return to fighting was "fully coordinated" with Washington.

A State Department spokesperson said that "Hamas bears total responsibility... for the resumption of hostilities".

The United Nations and countries around the world condemned the strikes, while the families of Israeli hostages pleaded with Prime Minister Benjamin Netanyahu to halt the violence, fearing for the fate of their loved ones.

UN chief Antonio Guterres said Gazans were being subjected to an "intolerable level of suffering" with the strikes.

- 'Complete destruction' -

Netanyahu's office said Tuesday's operation was ordered after "Hamas's repeated refusal to release our hostages".

Hamas said Israel had "decided to overturn the ceasefire agreement" brokered by US, Qatari and Egyptian mediators, and warned that the resumption of violence would "impose a death sentence" on the remaining hostages.

The group's leader, Sami Abu Zuhri, told AFP the aim of the strikes was "to impose a surrender agreement, writing it in the blood of Gaza".

Defence Minister Israel Katz said that "Hamas must understand that the rules of the game have changed", threatening to unleash the Israeli military until the group's "complete destruction" if it did not immediately free the hostages.

Hamas said the head of its government in Gaza, Essam al-Dalis, was among several officials killed.

In the southern Gaza Strip, AFP footage showed people rushing stretchers with wounded people, including young children, to hospital. Bodies covered with white sheets were also taken to the hospital's mortuary.

- 'Shocking' -

The Gaza health ministry said the bodies of 413 people had been received by hospitals, adding "a number of victims are still under the rubble".

UNICEF spokeswoman Rosalia Bollen, speaking to AFP in southern Gaza, said the deaths include "dozens and dozens of children, with many more children wounded".

Medical facilities that "have already been decimated" by the war were now "overwhelmed", she added.

Families of Israeli hostages rallied in front of Netanyahu's office in Jerusalem, and a campaign group accused the government of causing "the explosion of the ceasefire, which could sacrifice their family members".

Britain, France and Germany called for the renewed hostilities to end.

"The images of burning tents in refugee camps are shocking. Fleeing children and internally displaced persons must never be used as leverage in negotiations," said Berlin's Foreign Minister Annalena Baerbock.

Hamas backer Iran denounced the wave of attacks as a "continuation of the genocide" in the Palestinian territories, while Russia and China warned against an escalation.

Egypt, Qatar, Jordan and Turkey also condemned the violence.

Egyptian President Abdel Fattah al-Sisi said the strikes were part of "deliberate efforts to make the Gaza Strip uninhabitable and force the Palestinians into displacement".

Trump has floated a proposal to move Palestinians out of Gaza, an idea that was rejected by Palestinians and governments in the region and beyond, but embraced by some Israeli politicians.

Hours after the wave of strikes began, Netanyahu's Likud movement said that a far-right party that had quit the government in January in protest of the Gaza truce would rejoin.

- Attack from Yemen -

The ceasefire in Gaza took effect on January 19, largely halting the war triggered by Hamas's October 7, 2023 attack on Israel.

That first phase of the deal ended in early March after numerous exchanges of Israeli hostages and Palestinian prisoners.

But the two sides have been unable to agree on the next steps, with Hamas demanding negotiations for a second phase, which should lead to a lasting ceasefire.

Israel had sought to extend the first phase, cutting off aid and electricity to Gaza over the deadlock.

Hamas's 2023 attack resulted in 1,218 deaths, mostly civilians, while Israel's retaliation in Gaza has killed at least 48,577 people, also mostly civilians, according to figures from the two sides.

Of the 251 hostages seized during the attack, 58 are still in Gaza, including 34 the Israeli military says are dead.

On Tuesday evening, the Israeli military said it had intercepted a missile launched from Yemen, whose Iran-backed Huthi rebels have pursued a campaign of attacks against Israel and shipping in the Red Sea in what they say is solidarity with the Palestinians.