The Israel Defense Forces (IDF) said on Tuesday it launched a "large-scale ground operation" in Gaza City on the night of September 15, as local officials reported heavy strikes in the biggest urban center that left dozens dead or missing.

The army said it had urged civilians to leave in recent days, but hundreds of thousands of the city's roughly one million residents – many already displaced multiple times – are believed to remain. Many residents indicated that the high cost of moving, the perilous journey, and immobility caused by illness or disability were the main reasons they remained in the city instead of fleeing.

WAFA, the official Palestinian news agency, said at least 53 Palestinians were killed by Israeli gunfire and airstrikes across the Gaza Strip since dawn, most of them in Gaza City.

According to an IDF spokesperson, troops advanced "gradually, with heavy fire to support the forces." Around 10 p.m., units from the 98th and 162nd Armored Divisions entered Gaza City under the cover of extensive airstrikes, with the 36th Division expected to join in the coming days to "encircle Gaza City from all sides."

The military said about 130,000 reservists are being mobilized, a process due to be completed within days.

Israel's Prime Minister Benjamin Netanyahu, speaking at the opening of his corruption trial in Tel Aviv, called the operation "intense" and said, "The state of Israel is at a critical stage in this struggle, and there are consequences." His plan to seize control of Gaza City, announced last month, has drawn international criticism.

Israel has massed forces for weeks in preparation for what Netanyahu and other officials have described as a major ground and air offensive to "conquer" Gaza City.

Military spokesman Avichay Adraee on social media platform X called Gaza City "a dangerous combat zone," warning civilians that staying "puts you at risk." He said the IDF had begun dismantling Hamas infrastructure and noted that more than 40 percent of residents had fled in recent weeks.

Defense Minister Israel Katz added on Tuesday, "Gaza is burning," vowing that the military would strike Hamas "with an iron fist. We will not relent, and we will not turn back until the mission is completed."

U.S. offers support, EU plans new sanctions

In launching the assault, Israel's government defied European leaders threatening sanctions and ignored warnings from some of its own military commanders that the operation could be a costly mistake. Meanwhile, Washington appeared to offer its backing, conveyed by U.S. Secretary of State Marco Rubio.

Hours before the escalation, Rubio offered backing at a press conference alongside Netanyahu. While the U.S. wished for a diplomatic end to the war, "we have to be prepared for the possibility that's not going to happen", he said.

In Brussels, a spokesperson for the European Commission said the EU would agree on Wednesday to impose new sanctions on Israel, including the suspension of certain trade provisions.

"The EU has consistently urged Israel not to intensify its operation in Gaza City," EU spokesperson Anouar El Anouni said. "A military intervention will lead to more destruction, more death and more displacement, and we have been clear that this will also aggravate the already catastrophic humanitarian situation and also endangers the lives of hostages," he said.

British Foreign Secretary Yvette Cooper called the assault "reckless and appalling" and called for an immediate ceasefire. Britain and France are expected this month to become the first major Western powers to recognize Palestinian independence.

The United Nations High Commissioner for Human Rights Volker Türk on Tuesday condemned Israel's ground assault on Gaza City as "totally and utterly unacceptable" and demanded an end to the "carnage."

Israeli opposition leader Yair Lapid criticized Netanyahu's decision to launch the invasion, calling his conduct "amateurish and sloppy." In an interview with Ynet, Lapid questioned the lack of a clear plan for Gaza's future, the fate of Israeli hostages, and an exit strategy from the war.

Separately, the Palestinian Islamic Resistance Movement (Hamas) on Tuesday criticized U.S. President Donald Trump's remarks on the situation in Gaza and Israeli hostages, calling them "blatant bias toward Israeli propaganda" and "double standards."

(With input from agencies)