Israel said it launched an expanded ground assault on Gaza City on Tuesday in defiance of international condemnation, as Palestinians fled the enclave’s largest urban area in waves amid escalating bombardment.

The long-anticipated incursion began on the outskirts of the city, Israeli officials said, where Israel’s military has accelerated its airstrikes and the destruction of high-rise towers over the last week.

“Gaza is burning,” Defense Minister Israel Katz wrote on Tuesday. He said the Israel Defense Forces (IDF) was “striking terror infrastructures” and working to secure “the release of the hostages and the defeat of Hamas.” Prime Minister Benjamin Netanyahu said Israel is at a “critical stage” in the war as it attacks Gaza City, which his government sees as one of the last remaining strongholds of Hamas.

The incursion comes as the United Nations and others have warned that the assault will worsen an already dire humanitarian crisis, with parts of the Gaza officially declared under famine. Approximately one million people – nearly half of the territory’s population – live in and around Gaza City. Israel has tried to force the local population to evacuate, but the IDF has said that only about 40% of people have left so far