Israel ordered residents out of the centre of Gaza's main southern city Khan Younis and intensified its ground operations, after the US vetoed the demand for a ceasefire at the UN Security Council.
Israel's Arabic-language spokesperson on Saturday posted a map on X highlighting six blocks of Khan Younis to evacuate "urgently".
In central Gaza, Israeli tank shelling resumed on Bureij and Maghazi refugee camps, residents said, while Palestinian health officials reported an Israeli air strike in Bureij.
World Health Organization Director-General Tedros Adhanom Ghebreyesus stressed that the impact of the Israel-Hamas conflict on Gaza's healthcare sector has been "catastrophic".
Speaking at an emergency board meeting, he said the conditions were ideal for the spread of deadly diseases.
Since a truce with Hamas in the two-month-old war collapsed on December 1, Israel has expanded its ground assault into the southern half of the Gaza Strip, pushing into Khan Younis, where residents reported fierce battles.
Both sides reported a surge in fighting in the north.
Israel's National Security Adviser Tzachi Hanegbi said its forces had killed at least 7,000 Hamas militants, without saying how that estimate was reached.
An official toll of deaths in Gaza from the Palestinian health ministry in the Hamas-run enclave exceeded 17,700, with many thousands missing and presumed dead under the rubble. The ministry has said about 40 per cent of deaths were of children under 18.
Israel launched its campaign on Gaza, after Hamas fighters burst into Israeli towns on October 7, killing 1,200 people and taking 240 hostage.
Israeli forces say they are limiting civilian casualties by providing maps showing safe areas, and blame Hamas for harming civilians by hiding among them, which the fighters deny.