Israel ordered residents out of the center of Gaza's main southern city Khan Younis on Saturday and pounded the length of the enclave, after the United States wielded its UN Security Council veto to shield its ally from a demand for a ceasefire.

Since a truce collapsed last week, Israel has expanded its ground assault into the southern half of the Gaza Strip by pushing into Khan Younis. Simultaneously, both sides have reported a surge in fighting in the north.

Israel said its campaign was making progress. National Security Advisor Tzachi Hanegbi said Israeli forces had killed at least 7,000 Hamas militants so far, without saying how that estimate was reached, and military chief Lieutenant-General Herzi Halevi told soldiers "we need to press harder."

An official toll of all deaths in Gaza compiled by the Palestinian Health Ministry in the Hamas-run enclave exceeded 17,700 on Saturday, with many thousands more missing and presumed dead under the rubble. The ministry has previously said about 40 percent of deaths were of children under 18.

Israel's Arabic-language spokesperson posted a map on X highlighting six numbered blocks of Khan Younis that residents were told to evacuate "urgently." They included parts of the city center that had not been subject to such orders before.

Israel issued similar warnings before storming eastern parts of the city and residents said they feared new evacuation orders heralded a further assault.

"It might be a matter of time before they act against our area too. We have been hearing bombing all night," said Zainab Khalil, 57, displaced with 30 of her relatives and friends in Khan Younis near Jalal street where troops told people to leave.

"We don't sleep at night, we stay awake, we try to put the children to sleep and we stay up fearing the place would be bombed and we'll have to run carrying the children out. During the day begins another tragedy, and that is: how to feed the children?"

In central Gaza, Israeli tank shelling resumed on Bureij and Maghazi refugee camps, local residents said. According to Palestinian health officials, seven Palestinians were killed in an Israeli air strike in Bureij. Israeli forces could not immediately be reached for comment.

Meanwhile, with food and medical supplies scarce, a senior UN World Food Program official said a new system could bring aid into Gaza through the Kerem Shalom crossing with Israel, potentially allowing imports to ramp up. However, Israel has not yet agreed to open the crossing.

The vast majority of Gaza's 2.3 million residents have already been forced from their homes, many fleeing several times. With fighting raging across the length of the territory, residents and UN agencies say there is now effectively nowhere safe to go, though Israel disputes this.

In Khan Younis, the dead and wounded arrived through the night at the overwhelmed Nasser hospital.

A medic ran out of an ambulance with the limp body of a small girl in a pink tracksuit. Inside, wounded children wailed and writhed on the tile floor as nurses raced to comfort them. Outside, bodies were lined up in white shrouds.