An Israeli military official said the Israeli Defence Forces had begun the main stage of their ground operation into Gaza City, the main urban centre in the enclave, where Israel has ordered hundred of thousands of residents to flee.
The military gave few initial details but said its troops had begun "dismantling Hamas terrorist infrastructure in Gaza City". Residents should leave.
"Gaza is burning," Israeli Defence Minister Israel Katz said in a post on X.
"The IDF strikes with an iron fist at the terrorist infrastructure and IDF soldiers are fighting bravely to create the conditions for the release of the hostages and the defeat of Hamas."
Israeli warplanes carried out near-continuous strikes on Gaza City overnight, the Palestinian news agency WAFA reported.
Residents said bombardment of the city had been ramped up dramatically over the past two days, with heavier explosions that destroyed dozens of homes, and naval boats joining tanks and planes in the bombardment of the coast.
"We have launched a significant operation in Gaza," Israeli Prime Minister Benjamin Netanyahu said at the start of testimony in court in an ongoing corruption trial.
US Secretary of State Marco Rubio offered apparent backing for the Israeli government's decision to abandon ceasefire talks and use force to smash Hamas.
"Well, as you saw the Israelis have begun to take operations there. So we think we have a very short window of time in which a deal can happen," Rubio told journalists before leaving Israel for Qatar.
"Our preference, our No.1 choice, is that this ends through a negotiated settlement," he added, while acknowledging the dangers an intensifed military campaign posed to Gaza.
"The only thing worse than a war is a protracted one that goes on forever and ever," Rubio said.
"At some point, this has to end. At some point, Hamas has to be defanged, and we hope it can happen through a negotiation. But I think time, unfortunately, is running out."
Gaza health officials reported at least 24 people killed, most of them in Gaza City, in the early hours of the assault.
The Shifa Hospital in Gaza City received the bodies of 12 people killed in a strike that hit multiple houses in the western part of the city, with another 90 wounded arriving at the facility in recent hours, said Dr Rami Mhanna, hospital managing director.
"It was a heavy night," said Radwan Hayder, a Gaza City resident sheltering near the Shifa Hospital.
Earlier Netanyahu said he did not rule out further strikes on Hamas leaders "wherever they are", as the heads of Arab and Islamic states held a summit to show support for Qatar after Israel's attack on the Gulf state last week.
The September 9 strike targeting leaders of the Palestinian militant group in Doha marked a significant escalation of Israeli military action in a region shaken by conflict since the Hamas-led October 7, 2023 attacks that ignited the Gaza war.
Washington has said it was not warned in advance before Israel attacked Qatar, which houses the biggest US military base in the Middle East. President Donald Trump said on Sunday Israel had to be "very, very careful".
"They have to do something about Hamas, but Qatar has been a great ally to the United States," Trump said.
Hamas has said the Israeli strike on Qatar killed five of its members, including a son of its exiled Gaza chief, but its leadership survived. Qatar says one of its security agents also died.
With a broad spectrum of states attending - including Iran, Turkey and Saudi Arabia - it wasn't immediately clear what measures might be agreed at the summit.
A statement was expected to say the Israeli attack and other "hostile acts" threaten coexistence and efforts to normalise ties in the region, according to a draft seen by Reuters.
with DPA and AP
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