Israel battled Hamas militants in the Gaza Strip's biggest cities on Thursday and said it had attacked dozens of targets, leaving Palestinians struggling to survive in a situation the United Nations described as "apocalyptic".

Gazans crammed into Rafah on the border with Egypt, heeding Israeli leaflets and messages saying that they would be safe in the city.

But more than 20 people were killed in apartments there on Wednesday sheltering displaced civilians from the north, said Eyad al-Hobi, a relative of some of those killed.

"All apartments in the building suffered serious damage," he said as people brought out two apparently lifeless children.

Another relative, Bassam al-Hobi, said the building had been hit by three rockets.

"They targeted women and children, as you can see, and the guests who were told the south would be safe," he said, gesturing to bodies wrapped in white cloth, some small, lined up on the ground and surrounded by mourners.

Elsewhere in Rafah, medics said four people had been killed while traveling in a rickshaw on Thursday.

Israel said militants had fired at least one rocket from Rafah and that its forces had killed a number of gunmen in southern Gaza's largest city, Khan Younis, including two militants who emerged firing from a tunnel.

Hamas' armed wing, al-Qassam Brigades, said combat was fierce after troops entered the heart of Khan Younis on Wednesday in a new phase of the war, which is now entering its third month.

Ambulances and relatives rushed people, including women and children, into the Nasser hospital in Khan Younis, but even the floor space inside was already full. Two badly wounded children lay on one trolley and a dust-covered and bloodstained young boy lay screaming among the patients on the floor.

"The injuries are very severe," said doctor Mohamed Matar. "The situation is catastrophic in all senses of the word...We can't treat the injured in this state."

Residents said several Israeli air strikes had hit the city and there was non-stop tank fire on its eastern side. In central Gaza, medics said four people had been killed in a house in Nusseirat refugee camp overnight.

Those who escape violence face an increasingly desperate struggle to survive.

Hundreds of people packed a road in central city of Deir al-Balah, waiting for food outside a U.N. compound that had yet to open, a video posted by Ramy Abdu, founder of the Euro-Mediterranean Human Rights Monitor, showed.

The U.N. Palestinian Refugee Agency (UNRWA) said 1.9 million people - 85 percent of Gaza's population - had been displaced, its shelters were four times over capacity, and there was not enough aid to meet "the overwhelming needs".

Egypt said it was working to accelerate the delivery of aid but would never allow the emptying of the Gaza Strip of its residents as they are pushed southwards towards its border.

The Palestinian death toll reported by Gaza medics in the eight weeks of warfare was at 16,015, including 43 reported by one hospital on Tuesday and 73 by another on Wednesday.

Gaza's health ministry has not released overall casualty figures since Monday, and it and the U.N. say hundreds of people are unaccounted for under rubble.

BOMBING AND GUNBATTLES

Residents in Gaza City in the north reported all-night bombing and fierce gunbattles in Shejaia, east of the centre, and the Jabalia refugee camp further north, where Qatar-based Al Jazeera Media Network said 22 relatives of its Gaza correspondent Moamen Al-Sharafi had been killed.

Another district, Sabra, was also bombed, local people said.

Israel said it had raided a Hamas compound in Jabalia, killing several gunmen and found tunnels, a training area and weapons. In Shejaia, the armed wing of Hamas-allied Islamic Jihad, Al-Quds Brigades, said fighters had hit Israeli tanks.

In Khan Younis, Israeli forces had encircled the house of Hamas leader Yahya Al-Sinwar, Prime Minister Benjamin Netanyahu said on Wednesday evening.

"His home may not be his castle, and he can escape, but it's only a matter of time before we get him," he said.

The surprise incursion by Hamas fighters who rampaged through Israeli towns on Oct 7. killed 1,200 people, with 240 people taken hostage, according to Israel's tally.

The Israeli military says 88 soldiers have been killed in ground incursions into Gaza that began on Oct. 20 and that about a third of the reported Palestinian toll consisted of combatants, without saying how that estimate was reached.

In Geneva, the U.N. human rights chief said the situation in Gaza was "apocalyptic" with the risk that serious rights violations were being committed by both sides.

NO AID IN MOST OF GAZA FOR FOUR DAYS

The U.N. has been unable to distribute aid in any part of the Gaza Strip except for the area around Rafah for the past four days, it said in its daily humanitarian report on Thursday.

In Rafah, about 13 km (8 miles) south of Khan Younis, most of those displaced from elsewhere were sleeping rough as the U.N. had only managed to hand out a few hundred tents, the U.N. humanitarian office said on Wednesday.

Displaced civilians were also fleeing to the desolate area of Al Mawasi on Gaza's southern Mediterranean coast. Israel had declared the area safe but accused militants on Thursday of firing 12 rockets from there, leaving its status unclear.

The former Bedouin village lacks shelter, food and other necessities, according to refugee organisations.

A Palestinian who fled there, Ibrahim Mahram, said five families were sharing a tent.

"We suffered from the war of cannons and escaped it to arrive at the war of starvation," he told Reuters. "We divide one tomato between all of us."

A senior Hamas official told Reuters mediators were still exploring opportunities for a truce and reiterated its demand that Israel cease its attacks.

The U.N. Security Council received a UAE-drafted resolution on Wednesday demanding an "immediate humanitarian ceasefire", with a vote sought on Friday.