Israel unleashed a long-threatened ground assault on Gaza City on Tuesday, declaring "Gaza is burning" as Palestinians there described the most intense bombardment they had faced in two years of war.

An Israel Defence Forces official said ground troops were moving deeper into the enclave's main city, and that the number of soldiers would rise in coming days to confront up to 3,000 Hamas combatants the IDF believes are still in the city.

"Gaza is burning," Defence Minister Israel Katz posted on X. "The IDF strikes with an iron fist at the terrorist infrastructure and IDF soldiers are fighting bravely to create the conditions for the release of the hostages and the defeat of Hamas."

In launching the assault, Israel's government defied European leaders threatening sanctions and warnings from even some of Israel's own military commanders that it could be a costly mistake.

US President Donald Trump sided with Israel, telling reporters at the White House that Hamas would have "hell to pay" if it used hostages as human shields during the assault.

Israeli Prime Minister Benjamin Netanyahu said Trump had invited him to the White House in two weeks, after both men have addressed an annual gathering of world leaders at the United Nations General Assembly.

In the latest expression of international alarm, a United Nations Commission of Inquiry concluded that Israel had committed genocide in Gaza. Israel called the assessment "scandalous" and "fake".

United Nations Secretary-General Antonio Guterres said that what is happening in Gaza was horrendous and that the war in the Palestinian territory was morally, politically and legally intolerable.

Palestinian local health authorities said an Israeli strike had hit a vehicle carrying displaced people fleeing south near the coastal road in Gaza City. The latest deaths take Tuesday’s toll to at least 75, most of them in Gaza City.

The Israeli military did not immediately comment on the strike.

Where a missile had destroyed two multi-storey residential buildings during the night, people clambered over an immense mound of dislocated concrete to pry out victims, footage obtained by Reuters showed.

A woman cried as a small child's body was pulled from the wreckage, hastily wrapped in a green blanket and carried away.

Israel renewed calls on civilians to leave, and columns of Palestinians streamed towards the south and west in donkey carts, rickshaws, heavily laden vehicles or on foot.

Some Gaza residents were staying put, too poor to secure a tent and transport or because there was nowhere safe to go.

"It is like escaping from death towards death, so we are not leaving," said Um Mohammad, a woman living in the suburb of Sabra, under aerial and ground fire for days.

The IDF said it estimated that 40 percent of people in Gaza City had left. Hamas said 350,000 had left their homes in the eastern parts of the city, heading to displacement shelters in its central or western areas, while another 175,000 people had fled the city altogether, heading south.

In Brussels, a spokesperson for the EU executive said it would agree on Wednesday to impose new sanctions on Israel, including suspending certain trade provisions.

Netanyahu told a press conference on Wednesday that if there was one lesson learned from the Gaza war it was that Israel needs to create an "independent weapons industry" that can "withstand international constraints".

Some Israeli military commanders have expressed concern that the Gaza City assault could endanger remaining hostages held by Hamas or be a "death trap" for troops.

Chief of Staff Eyal Zamir, at a meeting Netanyahu convened late on Sunday with security chiefs, urged the prime minister to pursue a ceasefire deal, according to three Israeli officials, two of whom were in the meeting and one of whom was briefed on its details. (Reuters)