Israeli forces in war-ravaged Gaza City pushed on with a major offensive on Tuesday that has again displaced Palestinians, as UN experts said children were dying in a "starvation campaign."

Troops, tanks and fighter jets swooped on Gaza's biggest urban area on the eve of new contacts in Qatar aiming for an eventual hostage-prisoner exchange and a truce in the war raging into its 10th month.

CIA director William Burns and Israel's Mossad chief David Barnea are due to travel to Qatar on Wednesday, after Burns held talks with Egypt's President Abdel Fattah al-Sisi in Cairo.

Hamas, whose October 7 attack triggered the war, has accused Israeli Prime Minister Benjamin Netanyahu of deliberately escalating fighting in Gaza City and Rafah, in the territory's south, to thwart an agreement.

The Islamist group's Qatar-based political chief Ismail Haniyeh said he had made "urgent contact" with mediators, warning that the "catastrophic consequences" of the latest battles could "reset the negotiation process to square one."

Hamas's armed wing, the Ezzedine Al-Qassam Brigades, described the fighting in Gaza City in recent days as "the most intense in months."

Militants were fighting with rockets, mortars and explosives, it said.

After almost two weeks of battles in Gaza City's eastern Shujaiya district, Israeli forces have extended the fighting into the city's east, west and south.

Residents reported helicopter strikes, "explosions and numerous gun battles" in the city's southwest.

Israel's military said it was pursuing Palestinian militants in Gaza City, six months after it said it had dismantled Hamas's "military framework" in the territory's north.

Aircraft struck the city as troops were engaged in "close-quarters combat," seizing weapons and destroying tunnels, the military said, reporting "dozens" of militants killed.

The United Nations said tens of thousands of civilians have been affected by the surge in fighting since the first of three evacuation orders for Gaza City was declared on June 27.

Thousands were seen marching down dusty roads past bombed-out buildings, with mothers carrying babies and others packing belongings onto donkey carts.

The UN Human Rights Office said it was "appalled" at the way civilians, many of whom have been displaced multiple times, have been ordered to head to areas where "military operations are ongoing and where civilians continue to be killed and injured."

Gaza City residents have now been told to move south to Deir al-Balah, which the UN office said "is already seriously overcrowded" with displaced Palestinians.

Separately, independent UN rights experts accused Israel of carrying out a "targeted starvation campaign" that has resulted in the deaths of Gazan children.

"Israel's intentional and targeted starvation campaign against the Palestinian people is a form of genocidal violence," the experts said in a statement.

"Thirty-four Palestinians have died from malnutrition since October 7, the majority being children," said the experts, who are appointed by the UN Human Rights Council but who do not speak on behalf of the United Nations.

Israel's mission to the UN in Geneva accused the panel's members of "spreading misinformation" and "supporting Hamas propaganda."

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

The militants 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 38,243 people in Gaza, also mostly civilians, according to figures from the Hamas-run territory's health ministry.

The toll includes at least 50 killed over the past 24 hours, said a ministry statement on Tuesday.

Qatar and Egypt, supported by the United States, have been engaged in months of behind-the-scenes contacts to start truce talks and organise an exchange of hostages for Palestinian prisoners held in Israeli jails.

Hamas has signalled that it would drop its insistence on a "complete" ceasefire – which Israel had repeatedly rejected – as a condition for starting talks.

Netanyahu has set out conditions for talks, including that "any deal will allow Israel to return and fight until all the goals of the war are achieved," including the destruction of Hamas.

As the Gaza war has raged on, Israel has also exchanged regular cross-border fire with Lebanon's Hezbollah, allies of Hamas, heightening fears of an all-out war.

Hezbollah on Tuesday released a video showing aerial surveillance footage it said was taken over intelligence and military positions in the Israeli-annexed Syrian Golan Heights.

The release came after an Israeli strike killed a senior Hezbollah commander last week. The group's chief Hassan Nasrallah was expected to make a televised address on Wednesday at an event commemorating the slain fighter.

Israeli Foreign Minister Israel Katz told Nasrallah on social media to "stop the threats and violence," and "withdraw" Hezbollah forces from the border area, in line with a 2006 UN Security Council resolution that ended their latest major war.

The Lebanese government has supported the implementation of Resolution 1701, which called for armed personnel to pull back north of the Litani River, some 30 kilometres from the border with Israel.

If a full-blown conflict breaks out, Israel's top diplomat said Nasrallah "will be considered the destroyer of Lebanon." (AFP)