CAIRO: Israeli forces pushed deep into the ruins of Gaza's northern edge on Monday to recapture an area where they had claimed to have defeated Hamas months ago, while at the opposite end of the enclave tanks and troops pushed across a highway into Rafah.

With some of the most intense fighting for weeks now taking place on both the northern and southern edges of Gaza, hundreds of thousands of Palestinians have again taken flight, and aid groups warn that a humanitarian crisis could sharply worsen.

Israel described its latest return to the north, where it pulled out most of its troops five months ago, as part of a "mop-up" stage of the war to prevent fighters from returning, and said such operations had always been part of its plan. Palestinians say the need to keep fighting amid the ruins of previous battles is proof Israel's military objectives are unattainable.

In sprawling Jabalia, the biggest of Gaza's eight camps built 75 years ago to house Palestinian refugees from what is now Israel, tanks pushed towards the heart of the district. Residents said tank shells were landing at the centre of the camp and air strikes had destroyed clusters of houses.

Thick clouds of black smoke from explosions could be seen rising over northern Gaza from the Israeli border on Sunday.

Israeli troops are seeking to wipe out Hamas, which has said it is committed to Israel's destruction. The  group burst into Israel on Oct. 7, killing 1,200 and taking more than 250 hostages, by Israeli tallies.

The Palestinian death toll in the war has now surpassed 35,000, according to Gaza health officials who fear many more bodies are lost under the rubble. The fighting has laid waste to the coastal enclave and caused a deep humanitarian crisis, with the Gaza health ministry warning in a statement on Monday that the medical system is on the verge of collapse due to a shortage of fuel to power generators and ambulances.

Palestinian health officials on Monday said they had so far recovered 20 bodies of Palestinians killed in the overnight air strikes on Jabalia, while dozens were injured.

At the opposite end of Gaza in Rafah, against the border fence with Egypt, Israel stepped up aerial and ground bombardments on the eastern areas of the city, killing people in an air strike on a house in the Brazil neighbourhood.

Israel ordered residents out of the east of the city last week, and extended that order to central areas in recent days, sending hundreds of thousands of people, most of whom are already displaced, fleeing for new shelters.

Residents said Israeli air and ground bombardments were intensifying and tanks had cut off the main north-south Salahuddin Road that divides the eastern part of the city from the central area.

"The tanks cut the Saladuddin road east of the city, the forces are now in the southeast side, building up near the built-up area, the situation is dreadful and the sounds of explosions never stopped," said Bassam, 57, from the Shaboura neighbourhood in Rafah.

"People continue to leave Rafah, even far away near the western areas as no place looks safe now and also because people do not want to escape at the last minute should tanks make sudden incursions and moving out becomes too late," he told Reuters via a chat app.

UNRWA, the main United Nations aid agency in Gaza, estimated that about 360,000 people had fled the southern city since the Israeli military gave its first evacuation order a week ago.

BOMB SHIPMENT ON HOLD

The assault on Rafah has caused one of the biggest splits in generations between Israel and its main ally the United States, which put some deliveries of weapons on hold for the first time since the war began. Washington has said Israel must not assault Rafah without a plan in place to protect civilians there, which it has yet to see.

Jack Lew, the U.S. ambassador to Israel, signalled on Sunday that the Rafah incursion was still on a scale that Washington considers acceptable.

"The president was clear in the interview he gave the other evening that what Israel has done so far hasn't crossed over into the area where our disagreements lie," Lew told Israel's Channel 12 TV, without elaborating on what that area entails.

"I'm hoping we don't end up with real disagreement."

Hamas' armed wing said its fighters were engaged in gun battles with Israeli forces in one of the streets east of Rafah, and in the east of Jabalia.

In Israel, the military sounded sirens several times in areas near Gaza, warning of potential Palestinian cross-border rocket and or mortar launches.

Hamas and the armed wing of Islamic Jihad said in a joint statement that they fired mortar bombs against Israeli forces massing up inside the Rafah crossing, the sole checkpoint linking Gaza to Egypt, which Israel captured last week.

Late on Saturday, the Israeli military said forces operating in Jabalia were preventing Hamas, which rules Gaza, from re-establishing its military capabilities there.

"They were bombing everywhere, including near schools that are housing people who lost their houses," Jabalia resident Saed, 45, told Reuters via a chat app on Sunday. "War is restarting, this is how it looks in Jabalia."