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 conflict.

An Israel Defense 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," Defense 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.

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. Among them, Chief of Staff Eyal Zamir urged Israeli Prime Minister Benjamin Netanyahu to pursue a ceasefire deal.

'Morally, politically and legally intolerable'

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 is horrendous and that the war in the Palestinian territory is morally, politically and legally intolerable.

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.

Nearly 48,000 Palestinians have fled southward in the last two days from the Israeli military's ground offensive in Gaza City, UN humanitarians said on Tuesday. 

The UN Office for the Coordination of Humanitarian Affairs (OCHA) said that between mid-August and Monday, its partners observed more than 190,000 such movements, with many people traveling on foot due to the extremely high cost of transport.

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," Um Mohammad, a woman living in the suburb of Sabra, under aerial and ground fire for days, told Reuters.

Besides, foreign ministers from South Africa and 15 other countries on Tuesday expressed concern over the safety of the international Global Sumud Flotilla heading to Gaza in a joint statement.

The statement called on all parties to respect international law and refrain from any unlawful or violent act against the flotilla, warning that violations, including attacks in international waters or illegal detention, would lead to accountability.

The flotilla, a civil society initiative, aims to deliver humanitarian aid to the Gaza Strip. It departed from Barcelona, Spain on September 1, and is expected to arrive in Gaza in mid-September.

(With input from agencies)