The United Nations has warned Israel against the "forcible transfer of civilians" in Gaza, which could be in breach of international law.

The UN human rights office said on Tuesday that a legal temporary evacuation of civilians came with obligations on Israel, which, the Geneva-based agency said, it appears to have made no attempt to fulfil.

Israel has demanded that residents of north Gaza leave for the south, hoping to clear the area of civilians in preparation for a perilous urban ground assault.

"We have grave fears about the toll on civilians in the coming days. Military operations show no signs of abating," UN rights office spokesperson Ravina Shamdasani told a media briefing.

"International law requires that any lawful temporary evacuation by Israel, as the occupying power, of an area on the basis of the security of the population or imperative military reasons, must be accompanied by the provision of proper accommodation for all evacuees, undertaken under satisfactory conditions of hygiene, health, safety and nutrition."

'Completesiege'

"There appears to have been no attempt by Israel to ensure this for the 1.1 million civilians ordered to move," she added.

"We are concerned that this order combined with the imposition of a 'complete siege' on Gaza may not be considered as lawful temporary evacuation and would therefore amount to a forcible transfer of civilians -in breach of international law."

Gaza-based Hamas fighters broke through Israel's heavily fortified border on October 7.

Israel has responded with withering air strikes on Gaza and by deploying tens of thousands of troops to the border in preparation for a full-scale ground invasion.