Israeli air strikes and artillery bombardments have hit Khan Younis in the south of the Gaza Strip as Israel presses its offensive against Hamas militants with renewed force after the collapse of a truce in the almost two-month-old war.
Palestinian residents said on Saturday that houses and open areas had been hit and three mosques destroyed in Khan Younis in the past hours.
Columns of smoke were rising into the sky, Reuters journalists in the city said.
The Israeli military said that in the last 24 hours, combined attacks by its ground, air and naval forces had hit 400 militant targets and killed an unspecified number of Hamas fighters.
Hamas media said about 200 Palestinians had been killed since the end of the truce - adding to the more than 15,000 dead in Gaza since the start of the war, according to health authorities in the enclave.
The warring sides blamed each other for the collapse of the seven-day truce, during which Hamas had released hostages in exchange for Palestinian prisoners held in Israeli jails.
The United Nations said the fighting would worsen an extreme humanitarian emergency.
"Hell on earth has returned to Gaza," said Jens Laerke, spokesperson for the United Nations humanitarian office in Geneva.
The first aid trucks since the end of the truce entered through the Egyptian side of the Rafah crossing on Saturday, Egyptian security and Red Crescent sources told Reuters.
The conflict broke out on October 7 when Hamas militants crossed into southern Israel and killed 1200 people, mostly civilians, in a rampage against kibbutzim and other communities.
More than 200 hostages were taken back to Gaza.
Israel responded with a ferocious bombing campaign and a ground offensive which has destroyed large areas of Gaza and displaced hundreds of thousands of people in what has become the the bloodiest episode of the wider Israel-Palestinian conflict.
Friday saw intense bombing in Khan Younis and Rafah in the south, medics and witnesses said.
Displaced Gazans have been sheltering there because of fighting in the north of the densely populated enclave but residents said they feared Israel was preparing for ground troops to move on the south.
Leaflets dropped by Israel on eastern areas of Khan Younis ordered residents of four towns to evacuate - not to other areas in Khan Younis as in the past, but further south to Rafah.
"You have been warned," the leaflets said in Arabic.
The Israeli military said it had killed many squads of fighters in northern Gaza, including in a gunbattle at a mosque used by Islamic Jihad militants as a command post.
Signalling that it was deepening its grip on the north, it also instructed civilians in several districts of Gaza City to evacuate immediately.
In southern Israel, rocket sirens sounded early on Saturday in communities near the border with Gaza, but there were no reports of serious damage or casualties.
Reuters could not confirm the battlefield accounts.
The truce that started on November 24 had been extended twice but after seven days during which women, children and foreign hostages were freed as well as Palestinian prisoners, mediators failed to find a formula to release more.
Israel accused Hamas of refusing to release all the women it held.
A Palestinian official said the breakdown occurred over female Israeli soldiers.
Qatar, which has played a central mediating role, said negotiations were continuing with Israelis and Palestinians to restore the truce but Israel's renewed bombardment of Gaza had complicated matters.
An Israeli official in Washington said it was a "very high priority" to get as many hostages released as possible.
"And for that, under agreed terms, Israel is willing to give additional pauses," the official said.
"We can negotiate while we still fight."
US Vice President Kamala Harris, who will attend the COP28 climate summit in Dubai, was due to lay out American objectives for when the Israel-Hamas conflict ends.
However, Turkey's President Tayyip Erdogan said in Istanbul that the chance for peace in Gaza after the pause was lost for now due to what he called Israel's uncompromising approach.
Australian Associated Press