Strikes were reported in multiple locations early on Tuesday, including northern Gaza, Gaza City and the Deir al-Balah, Khan Younis and Rafah in central and southern Gaza Strip. Palestinian health ministry officials said many of the dead were children.
The military, which said it hit dozens of targets, said the strikes would continue for as long as necessary and would extend beyond air strikes.
The attacks were far wider in scale than the regular series of drone strikes the Israeli military has said it has conducted against individuals or small groups of suspected militants and follows weeks of failed efforts to agree an extension to the truce agreed on January 19.
Hamas said Israel had overturned the ceasefire agreement, leaving the fate of 59 hostages still held in Gaza uncertain.
Israeli Prime Minister Benjamin Netanyahu's office accused Hamas of "repeated refusal to release our hostages" and rejecting proposals from US President Donald Trump's Mideast envoy Steve Witkoff.
"Israel will, from now on, act against Hamas with increasing military strength," it said in a statement.
In Washington, a White House spokesperson said Israel had consulted the US administration before it carried out the strikes, which the military said targeted mid-level Hamas commanders and leadership officials as well as infrastructure belonging to the militant group.
Negotiating teams from Israel and Hamas had been in Doha as mediators from Egypt and Qatar sought to bridge the gap between the two sides following the end of an initial phase in the ceasefire, which saw 33 Israeli hostages and five Thais returned by militant groups in Gaza in exchange for some 2000 Palestinian prisoners.
With the backing of the United States, Israel had been pressing for the return of the remaining 59 hostages still held in Gaza in exchange for a longer-term truce that would have halted fighting until after the Muslim fasting month of Ramadan and the Jewish Passover holiday in April.
However, Hamas had been insisting on moving to negotiations for a permanent end to the war and a full withdrawal of Israeli forces from Gaza, in accordance with the terms of the original ceasefire agreement.
"We demand that the mediators hold Netanyahu and the Zionist occupation fully responsible for violating and overturning the agreement," the group said.
Each side has accused the other of failing to respect the terms of the January ceasefire agreement, and there were multiple hiccups during the course of the first phase. But until now, a full return to the fighting had been avoided.
Israel had blocked deliveries of aid from entering Gaza and had threatened on numerous occasions to resume fighting if Hamas did not agree to return the hostages it still holds.
Much of Gaza now lies in ruins after 15 months of fighting, which erupted on October 7, 2023 when thousands of Hamas-led gunmen attacked Israeli communities around the Gaza Strip, killing some 1200 people, according to Israeli tallies, and abducting 251 hostages into Gaza.
The Israeli campaign in response has killed more than 48,000 people, according to Palestinian health authorities, and destroyed much of the housing and infrastructure in the enclave, including the hospital system.
