Israel said on Saturday its war with Hamas had "entered a new phase" as the UN warned thousands more civilians could die from escalated ground operations.

Gaza has been relentlessly pounded by air and artillery strikes three weeks into a conflict sparked by the deadliest attack in Israel's history. Hamas gunmen stormed across the Gaza border on October 7, killing 1,400 people, mostly civilians, and seizing more than 220 hostages, according to Israeli officials.

The Hamas-controlled health ministry in Gaza said on Saturday Israeli strikes had killed 7,703 people, mainly civilians, with more than 3,500 of them children. The conflict is the fifth and deadliest in Gaza since Israel unilaterally withdrew troops and settlers from the Palestinian territory in 2005.

UN human rights chief Volker Turk warned of "the possibly catastrophic consequences of large-scale ground operations in Gaza", saying "thousands more civilians" could die.

The intense Israeli strikes against Hamas, the Islamist group that has ruled Gaza since 2007, coincided with ground operations and came as tens of thousands of troops massed along the Gaza border ahead of an expected full-blown invasion.

"We've entered a new phase in the war," Israeli Defence Minister Yoav Gallant said on Saturday. "Last night, the ground in Gaza shook. We attacked above the ground and below the ground. We attacked terror operatives of all ranks in every location."

Gaza's phone and internet services have been almost completely cut since Friday evening making it almost impossible to communicate with the outside world.

The head of the World Health Organization, Tedros Adhanom Ghebreyesus, said the blackout was blocking ambulances and evacuations of patients and denying people safe shelter. He and other aid agencies said they could not contact their staff, but a representative from the International Committees of the Red Cross and Red Crescent in Gaza got an audio message out.

Israeli fighter jets dropped leaflets over Gaza City Saturday warning residents that the area was now a "battlefield."

"Shelters in northern Gaza and Gaza governorate are not safe," the army said, urging residents to "evacuate immediately" to the south. 

Gaza Civil Defence spokesman Mahmud Bassal said "hundreds of buildings and houses were completely destroyed and thousands of other homes were damaged" in the overnight strikes.

Amid the rubble of destroyed buildings in Al-Shati refugee camp in the northern Gaza Strip, camp resident Alaa Mahdi likened Israel's bombardment to an "earthquake". He said: "If it were a natural earthquake, it would have been much easier than what happened last night."

Israeli fighter jets hit 150 "terror tunnels, underground combat spaces and additional underground infrastructure" and "several Hamas terrorists were killed", the army said. Hamas's armed wing, the Ezzedine al-Qassam Brigades, said it targeted Israeli forces in an area of northern Gaza near the border on Saturday.

The families of Israeli hostages held by Hamas demanded an immediate government explanation about their fate after the army's intensified strikes. The Hostages and Missing Families Forum, which represents some 229 people believed abducted by Hamas, said the war cabinet had failed to explain to relatives whether the ground operation endangered the captives' well-being.

"The families don't sleep, they want answers, they deserve answers," said Haim Rubinstein, 35, a spokesperson at an event in Tel Aviv on Saturday where hundreds gathered threatening to stage street protests if there was no response.

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The Israeli military also reported a new exchange of fire on the border with Lebanon on Saturday, the latest in what have been the most serious clashes on the border since 2006. Israel's neighbor Egypt said drones fell on the country on Friday.

"The region will become a ticking time bomb that impacts us all," warned Egypt's President Abdel Fattah al-Sisi.

EU foreign policy chief Josep Borrell demanded a "pause of hostilities" to allow aid into Gaza, after the UN General Assembly called for an "immediate humanitarian truce." The non-binding resolution on Friday received overwhelming support, but Israel and the U.S. criticized it for failing to mention Hamas.

Addressing a rally of several hundred thousand Palestinian supporters in Istanbul on Saturday, Turkish President Recep Tayyip Erdogan said "the main culprit behind the massacre unfolding in Gaza is the West."

Erdogan earlier said "Israel must immediately stop this madness and end its attacks."

UN chief Antonio Guterres said he regretted the "unprecedented escalation" of Israel's bombardment of Gaza and demanded an "immediate" ceasefire. He added: "I repeat my call for a humanitarian ceasefire in the Middle East, the unconditional release of all hostages, and the delivery of life-saving supplies at the scale needed. Everyone must assume their responsibilities. This is a moment of truth. History will judge us all."

Israel's bombardment has displaced more than 1.4 million people inside Gaza, according to the UN, while supplies of food, water and power to the crowded territory have been almost completely cut off.

A first tranche of aid was allowed on October 21, but only 84 have crossed in total, according to the UN, which says a daily average of 500 trucks had entered Gaza before the conflict.

Violence has also risen sharply in the occupied West Bank since the October 7 attacks, with more than 100 Palestinians killed and nearly 2,000 wounded, according to the UN.

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