Israel said on Saturday its war with Hamas had "entered a new phase" as the army relentlessly pounded Gaza three weeks into a conflict sparked by the deadliest attack in the country's history.

The United Nations warned thousands more civilians could die as Israel escalated ground operations in the Hamas-run Palestinian territory, while the European Union called for hostilities to stop to enable aid deliveries.

Israel unleashed its bombing campaign after 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 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.

Israeli forces also made limited ground incursions on Wednesday and Thursday.

"We've entered a new phase in the war," Israeli Defence Minister Yoav Gallant said in a video 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."

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.

The army already delivered similar warnings earlier in its campaign, but many who fled south ended up returning to their homes after failing to find refuge from Israeli bombing.

- 'A horror movie' -

Gaza Civil Defence spokesman Mahmud Bassal told AFP "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, Alaa Mahdi likened Israel's bombardment to an "earthquake".

"If it were a natural earthquake, it would have been much easier than what happened last night," he told AFP.

"What happened last night was more like a horror movie," Gaza City resident Jihad Mahdi told AFP.

"People in the streets have become lifeless bodies walking."

Hundreds of Hamas fighters crossed the Israeli border in vehicles, by air and sea on October 7, indiscriminately killing civilians in the streets, in their homes and at an outdoor rave party.

The unprecedented attack caught Israel off guard and exposed serious intelligence failings.

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.

Israeli warplanes flew overhead as booms were heard coming from Gaza, AFP journalists reported.

- 'Profound fear' -

The families of Israeli hostages held by Hamas demanded on Saturday 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.

On Thursday, the Ezzedine al-Qassam Brigades said "almost 50" hostages had been killed in Israeli bombing since October 7. AFP could not immediately verify the figure.

Tel Aviv resident Yael Leviel said reading about the Israeli operations offered "support and strength" as "all of us continue to live in profound fear".

"We cannot afford to lower our guard... our very existence is at stake," the 50-year-old social worker told AFP.

- Communications blackout -

Hamas said all internet connections and communications across Gaza had been cut.

AFP journalists in Gaza confirmed they were only able to communicate in limited areas where they could connect to Israeli networks across the border.

The Palestinian Red Crescent said the outage had disrupted ambulance services.

The UN humanitarian coordinator for the Palestinian territories, Lynne Hastings, said on X, formerly Twitter, that "hospitals & humanitarian operations can't continue without communications".

Between the bombardments and the fuel shortages, 12 of Gaza's 35 hospitals have been forced to close.

The UN agency for Palestinian refugees, UNRWA, said it has had to "significantly reduce its operations".

- 'Stop this madness' -

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 United States criticised 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" in a post on X.

UN chief Antonio Guterres said he regretted the "unprecedented escalation" of Israel's bombardment of Gaza and demanded an "immediate" ceasefire.

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.