CAIRO/GAZA - A new Israeli assault on Gaza on Monday threatened ceasefire talks at a crucial moment, the head of Hamas said, as Israeli tanks pressed into the heart of Gaza City and ordered residents out after a night of massive bombardment.

Residents said the airstrikes and artillery barrages were among the heaviest in nine months of conflict between Israeli forces and Hamas militants in the enclave. Thousands fled.

The assault unfolded as senior US officials were in the region pushing for a ceasefire after Hamas made major concessions last week. The militant group said the new offensive appeared intended to derail the talks and called for mediators to rein in Israel's Prime Minister Benjamin Netanyahu.

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The assault "could bring the negotiation process back to square one. Netanyahu and his army will bear full responsibility for the collapse of this path," Hamas quoted leader Ismail Haniyeh as saying.

The Gaza Civil Emergency Service said it believed dozens of people were killed but emergency teams were unable to reach them because of ongoing offensives

Gaza City, in the north of the Palestinian enclave, was one of Israel's first targets at the start of the war in October. But clashes with militants there have persisted and civilians have sought shelter elsewhere, adding to waves of displacement. Much of the city lies in ruins.

Residents said Gaza City neighborhoods were bombed through the night into the early morning hours of Monday. Several multi-storey buildings were destroyed, they said.

The Gaza Civil Emergency Service said it believed dozens of people were killed but emergency teams were unable to reach them because of ongoing offensives.

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Gaza residents said tanks advanced from at least three directions on Monday and reached the heart of Gaza City, backed by heavy Israeli fire from the air and ground.

That forced thousands of people out of their homes to look for safer shelter, which for many was impossible to find, and some slept on the roadside.

One tank thrust pushed people towards the western road near the Mediterranean, residents said.

"The enemy is behind us and the sea is in front of us, where we will we go?" said Abdel-Ghani, a Gaza City resident who did not give his full name.

"Tank shells and missiles from the planes are falling on the roads and houses like hell from a volcano. People are running in all directions, and no one knows where to go," Abdel-Ghani told Reuters via a chat app.

Later on Monday, Israel issued new evacuation orders for the Gaza City areas of Sabra, Rimal, Tel Al-Hawa and Daraj, telling people to head to Deir al-Balah in central Gaza

The Israeli military said fighters with Hamas and allied group Islamic Jihad were hiding behind civilian infrastructure to attack Israeli forces. Israel said it had taken more than 30 fighters out of action.

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Later on Monday, it issued new evacuation orders for the Gaza City areas of Sabra, Rimal, Tel Al-Hawa and Daraj, telling people to head to Deir al-Balah in central Gaza. The Israeli military said a route would be opened for civilians to evacuate.

The Palestinian Fatah Al-Aqsa Martyrs Brigades said they fired mortar bombs against Israeli forces during the raid in southwest Gaza City.

The war was triggered on Oct 7 when fighters led by Hamas, which controlled Gaza, attacked southern Israel, killing 1,200 people and taking around 250 hostages, according to Israeli figures.

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Since then at least 38,193 Palestinians have been killed in the military offensive and 87,903 have been wounded, Gaza's health ministry said in an update on Monday. A total of 40 Palestinians were killed in the past 24 hours, the ministry said in a statement.

Hopes for a pause

The new Israeli offensive comes as Egypt, Qatar and the United States stepped up efforts to mediate a ceasefire agreement between Israel and Hamas.

Hopes among Gaza residents of a pause in the fighting had revived after Hamas last week accepted a key part of a US ceasefire proposal, prompting an official on the Israeli negotiating team to say there was a real chance of a deal.

Mr. Prime Minister, this is not an absolute victory. This is total failure. We will not be part of a deal of surrender to Hamas.

Bezalel Smotrich, Israeli Finance Minister

Hamas has dropped a demand that Israel first commit to a permanent ceasefire before the Palestinian movement would sign an agreement. Instead, the militant group said it would allow negotiations to achieve that throughout the six-week first phase, a Hamas source told Reuters on Saturday.

READ MORE: OCHA: 250,000 people flee southern Gaza after Israeli evacuation order

Netanyahu insists the deal must not prevent Israel from resuming fighting until its war objectives are met. At the outset of the war, he pledged to annihilate Hamas.

Reaching a deal would jeopardize the coalition keeping Netanyahu in power, which includes far-right parties that have repeatedly vowed to quit if he ends the war too soon.

Finance Minister Bezalel Smotrich, from a party representing Israelis who settle on occupied Palestinian land, denounced the potential deal as "a defeat and humiliation of Israel".

"Mr. Prime Minister, this is not an absolute victory. This is total failure. We will not be part of a deal of surrender to Hamas," Smotrich told a meeting of his party on Monday.

A senior Hamas official said Hamas has dealt positively and flexibly with the Gaza ceasefire proposal and it is up to Netanyahu to make the next move

Israeli opposition leader Yair Lapid said he would back Netanyahu to stay in office if the prime minister faced losing power over committing to a deal. "I promised him a safety net and I will keep that promise," Lapid said.

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Meanwhile on Monday, a senior Hamas official said Hamas has dealt positively and flexibly with the Gaza ceasefire proposal and it is up to Netanyahu to make the next move.

"If the US administration wants to end the war, it should pressure Netanyahu to reach a ceasefire," Husam Badran, a Hamas Political Bureau member, said in a statement. "The Palestinian people are the victims in this conflict, while the Israeli army hides behind false claims of victimhood."

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Director of the US Central Intelligence Agency William Burns is expected to travel to Cairo this week to participate in negotiations regarding the prisoner exchange deal and ending the nine-month-long war in the Gaza Strip, Egyptian media reported on Sunday.

An Israeli delegation, including officials from agencies involved in the prisoner negotiations, was expected to arrive in Cairo on Sunday to resume ceasefire talks in Gaza, the media added.