Israel's army dropped thousands of leaflets over war-torn Gaza City on Wednesday urging all residents to flee a heavy offensive through the main city of the besieged Palestinian territory.

The leaflets, addressed to "everyone in Gaza City," set out designated escape routes and warned that the urban area, which had a pre-war population of over half a million, would "remain a dangerous combat zone."

The warning came as Israeli troops, backed by tanks and aircraft, have fought Hamas and Islamic Jihad militants in the heaviest combat the city has seen in months in the war raging since October 7.

In one operation, the army said it had killed militants and found weapons inside the long-vacated Gaza City headquarters of the UN agency for Palestinian refugees, UNRWA.

Elsewhere across Gaza, deadly strikes have hit four schools used as shelters in four days, sparking rebukes from France and Germany which both labelled the attacks "unacceptable."

"We call for these strikes to be fully investigated," said the French foreign ministry, highlighting a deadly strike on Tuesday on a school near the southern city of Khan Younis.

"It is unacceptable that schools, especially those housing civilians displaced by the fighting, should be targeted."

Heavy fighting also raged in Gaza's far-southern Rafah, where witnesses said that Israeli tanks had rumbled into the city centre and unleashed intense fire on buildings.

The latest fighting in Gaza has newly displaced 350,000 civilians, said UNRWA chief Philippe Lazzarini, who spoke before the leaflet drop and said "there is absolutely no safe space in Gaza."

One woman carrying her scant belongings through the ruins, Nimr al-Jamal, said that "this is the 12th time" her family has had to flee.

"How many times can we endure this? A thousand times? Where will we end up?"

The upsurge in fighting, bombardment and displacement came as talks were to resume in Qatar towards a truce and hostage release deal to end the war now grinding on into its 10th month.

An Israeli delegation led by Mossad chief David Barnea arrived in Doha, a source with knowledge of the negotiations said, speaking on condition of anonymity because of their sensitivity.

CIA director William Burns was also expected in the Qatari capital after holding talks in Cairo on Tuesday.

Israel's Prime Minister Benjamin Netanyahu meanwhile met US President Joe Biden's special envoy for the Middle East, Brett McGurk.

Netanyahu "emphasised his commitment" to a proposed truce plan, "as long as Israel's red lines are preserved," his office said.

The hawkish premier has repeatedly insisted that the war goals are to defeat Hamas and bring home all hostages.

The prime minister's office said "14,000 terrorists have been eliminated" since the beginning of the war.

Defence Minister Yoav Gallant separately told parliament that 60 percent of Hamas fighters had been "eliminated or wounded."

"The security establishment, and myself heading it, are determined to achieve the goals of the war and complete them," Gallant said.

The Israeli army said it was reviewing the Khan Younis area attack on Tuesday in which hospital sources said at least 29 people were killed in a school used as a displacement shelter.

Gaza's Hamas government said a "majority" of the dead were women and children.

Video footage showed the wounded being rushed to the nearby Nasser hospital, many screaming in pain, as relatives wailed in grief for the dead.

One wounded man, Osama Abu Daqqa, recounted that "suddenly the strike hit, people were injured and martyred and there was no one to help them."

Another survivor, Mohamed Sukkar, said those hit in the strike "were not part of the resistance nor were they armed, they were all civilians."

The military said a strike had hit near the school building and had targeted and killed a Hamas "terrorist" who had taken part in the October 7 attack.

It said it was "looking into the reports that civilians were harmed, adjacent to the Al Awda school," which it acknowledged was "near the location of the strike."

"The incident is under review."

Three previous strikes since Saturday on Gaza schools used by displaced Palestinians have killed a total of at least 20 people, said Gaza officials and rescue services.

Lazzarini wrote on social media platform X that "schools have gone from safe places of education and hope for children to overcrowded shelters and often ending up a place of death and misery."

Hamas's October 7 attack on southern Israel resulted in the deaths of 1,195 people, mostly civilians, according to a tally based on Israeli figures.

The militants also seized 251 hostages, 116 of whom remain in Gaza, including 42 the military says are dead.

Israel responded with a military offensive that has killed at least 38,295 people in Gaza, also mostly civilians, according to figures from the Hamas-run territory's health ministry.

Israel has also imposed a punishing siege on Gaza's 2.4 million people, eased only by sporadic aid deliveries.

Aid group Doctors Without Borders has warned of "critical" shortages of medical supplies in Gaza, with no resupply for more than two months.

Independent UN rights experts on Tuesday accused Israel of carrying out a "targeted starvation campaign," a claim strongly rejected by Israel.

Army spokesman David Baruch on Wednesday said that "the food is getting there, maybe not as much as everyone would want, but I don't see starvation." (AFP)