More than 15,200 people have been killed in the besieged Palestinian territory, according to Hamas, in more than eight weeks of combat and heavy bombardment.
Israeli air and artillery strikes hit Gaza's northern frontier with Israel, throwing thick clouds of smoke and dust into the sky.
At least one rocket salvo fired from the territory at Israel was intercepted by air defence systems, said AFP reporters in Sderot near Gaza.
The UN humanitarian agency OCHA said at least 160 Palestinian deaths were reported in two incidents in northern Gaza Saturday: the bombing of a six-storey building in Jabalia refugee camp, and of an entire block in a Gaza City neighbourhood.
Repeated bursts of heavy automatic weapons fire were heard over an AFPTV livecam.
Gaza's Hamas government on Saturday said 240 people had been killed since the week-long truce expired early Friday.
The war broke out when Hamas militants burst through Gaza's militarised border into Israel on October 7 and killed about 1,200 people, mostly civilians, while also taking around 240 hostages, according to Israeli authorities.
'No safe place'
After the unprecedented attacks, Israel unleashed a relentless air and ground campaign that has killed mostly women and children in Gaza, authorities there say.
Israel has vowed to destroy Hamas, while the militant group has ruled out more hostage releases until a permanent ceasefire is agreed.
"We have said it from day one: the price to pay for the release of Zionist prisoners will be the release of all our prisoners -– after a ceasefire," Saleh al-Arouri, deputy head of Hamas's politburo, said on Saturday evening.
Israel's army, after focusing on northern Gaza in recent weeks, has struck more targets in the territory's south and issued warnings to Palestinians trapped there to seek what it said would be safe zones, "if required".
United Nations High Commissioner for Human Rights Volker Turk warned that "as a result of Israel's conduct of hostilities and its orders for people to leave the north and parts of the south, hundreds of thousands are being confined into ever smaller areas in southern Gaza".
"I repeat, there is no safe place in Gaza," he said.
Hamas and another militant group, Islamic Jihad, announced "rocket barrages" against multiple Israeli cities and towns including Tel Aviv, and Israel said two of its soldiers had died in combat, the first since the truce ended.
In a summary issued on Sunday, the military said a drone strike had "eliminated" five Hamas militants.
Fighter jets and helicopters had also struck "tunnel shafts, command centres and weapons storage facilities" while naval forces hit Hamas vessels and weapons, it said.
At least seven people were killed in an Israeli bombing early Sunday near Gaza's southern border with Egypt, the Hamas government said.
'Too many' innocents killed
Israel's ally the United States, which provides it with billions of dollars in military aid annually, has intensified calls for the protection of Gaza's civilians.
"Too many innocent Palestinians have been killed," Vice President Kamala Harris told reporters at UN climate talks in Dubai.
In a new estimate, OCHA said around 1.8 million people in Gaza, roughly 75 percent of the population, had been displaced, many to overcrowded and unsanitary shelters.
Nasser hospital in the southern Gaza city of Khan Yunis overflowed with both the wounded and the dead.
Jumana Murad said her son Mohammad, 19, was killed as he tried to help women and children out of a tent inside a school.
"A piece of shrapnel hit him in the head," she told AFP before bursting into tears.
World Health Organization chief Tedros Adhanom Ghebreyesus said Sunday on X, formerly Twitter, that a WHO team visited Nasser hospital and found it with 1,000 patients, three times its capacity.
Some were being treated on the floor, in conditions "unimaginable for the provision of health care", he said.
Gazans are short of food, water and other essentials, and many homes have been destroyed but the aid reaching them is "a drop in the ocean of needs," said Adnan Abu Hasna, a spokesman for the UN agency for Palestinian refugees (UNRWA).
Surveillance
The seven-day truce, brokered by Qatar with support from Egypt and the United States, led to the release of 80 Israeli hostages in exchange for 240 Palestinian prisoners.
On Saturday Prime Minister Benjamin Netanyahu's office said Israeli negotiators were being withdrawn from Qatar "following the impasse in the negotiations".
There are still 137 hostages held in Gaza, Israel's army said.
Britain's defence ministry confirmed at the weekend that it will conduct surveillance flights over Israel and Gaza "in support of the ongoing hostage rescue activity".
Israeli hostages released from Gaza talked publicly on Saturday for the first time, urging their government to secure the release of the remaining captives.
Pope Francis on Sunday deplored "so much suffering in Gaza", and urged those involved to reach a new ceasefire deal.
'Total victory'
But Netanyahu said the war would continue "until we achieve all its aims" -- among them freeing the hostages -- and that soldiers had prepared during the truce "for total victory against Hamas".
"There is no way to win except by continuing the ground campaign," Netanyahu said, vowing that Israel would be "observing international law".
US Defense Secretary Lloyd Austin on Saturday drew on his experience fighting the Islamic State group in urging Israel to protect non-combatants.
"The lesson is that you can only win in urban warfare by protecting civilians," he told a forum in California.
At almost exactly the same time Israeli negotiators pulled out of the Qatar talks, bombs pounded the modern-looking yellow apartment blocks one by one.
Most Gazans are trapped, but an Egyptian border crossing, after a closure on Friday, reopened to enable 880 foreign and dual nationals to cross along with 13 injured people, the UN said.
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