The head of the United Nations agency for Palestinian refugees said Saturday that 800,000 people had been "forced to flee" Gaza's far-southern city of Rafah since Israel began military operations there this month.

"Nearly half of the population of Rafah or 800,000 people are on the road having been forced to flee since the Israeli forces started the military operation in the area on 6 May," UNRWA chief Philippe Lazzarini said in a post on social media site X.

Following the evacuation orders Gazans have fled to "the middle areas and Khan Younis including to destroyed buildings," he said.

"Every time, they are forced to leave behind the few belongings they have ....Every time, they have to start from scratch, all over again."

Israel has said the ground assault on Rafah was crucial to its fight against Palestinian militants, insisting it was the last Hamas stronghold in Gaza.

Before the operation began, Israel told hundreds of thousands of Gazans sheltering in some eastern parts of the city to leave, describing its operation there as "limited".

Israel's closest ally the United States expressed firm objections to the expansion of operations in Rafah, where 1.4 million Palestinian civilians were sheltering before the operation began.

Heavy clashes and bombardment rocked Rafah on Saturday, as Israel pressed an assault against Hamas militants.

An AFP reporter said air strikes and artillery shells pounded eastern parts of the city as warplanes criss-crossed above.

Lazzarini said people were fleeing to areas without water supplies or adequate sanitation.

Al-Mawasi, a 14 square kilometre town on the coast, as well as the central city of Deir el- Balah, were "crammed" with recently displaced people, Lazzarini added.