JERUSALEM, Sept. 16 (Xinhua) -- Israel's military said on Tuesday it launched a "large-scale ground operation" in Gaza City overnight, as local officials reported heavy strikes in the enclave's 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 previously told Xinhua 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.

A military spokesperson said troops were advancing "gradually, with heavy fire to support the forces." Around 10 p.m. (1900 GMT) on Monday, units from the 98th, 162nd, and 36th armored divisions entered Gaza City under the cover of extensive airstrikes, the spokesperson said, describing the overnight assaults as "staging points" for a wider push. More troops are 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.

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 wrote on social media platform X that Gaza City "is considered a dangerous combat zone" and warned civilians that staying "puts you at risk." He said the army had begun dismantling Hamas infrastructure and that more than 40 percent of residents had fled in recent weeks.

Videos posted online showed intense bombardment by aircraft and artillery, with flares and fires lighting up the skyline.

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

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, a United Nations commission accused Israel of "committing genocide" against Palestinians in Gaza, urging the international community to halt the campaign and hold senior officials accountable. Israel dismissed the allegations as "distorted, false and scandalous."