GENEVA The United Nations human rights office said on Tuesday that Israel's siege of Gaza and its evacuation order for the north of the enclave could amount to a forcible transfer of civilians and be in breach of international law.

Speaking to reporters in Geneva, Ravina Shamdasani, spokesperson for the U.N. human rights office, said Israel seemed to have made no effort to ensure the civilians temporarily evacuated in Gaza were provided with proper accommodation, as well as satisfactory conditions of hygiene, health, safety and nutrition.

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

"Those who managed to comply with the Israeli authorities' order to evacuate are now trapped in the south of the Gaza Strip, with scant shelter, fast-depleting food supplies, little or no access to clean water, sanitation, medicine and other basic needs."

The term "forcible transfer" describes the forced relocation of civilian populations and it is a crime against humanity punishable by the International Criminal Court (ICC).

In separate comments, the World Food Programme said its food supplies in Gaza were running low but that it was stockpiling supplies in the Egyptian city of Al-Arish nearby.

Abeer Etefa, WFP Regional Communications Lead for the Middle East and North Africa, said they hoped "to cross as soon as the border access is granted."

"We call for unimpeded access, safe passage to desperately needed humanitarian supplies to Gaza," she said.

Trucks carrying supplies headed towards the Rafah crossing in Egypt, the only access point to the enclave outside of Israel's control, though it was not certain whether they would be able to cross.

Fuel reserves at all hospitals across Gaza are expected to last for an additional 24 hours only, according to the U.N. Palestinian agency UNRWA.

"The shutdown of backup generators would place the lives of thousands of patients at serious risk," it said.

(Reporting by Gabrielle Tétrault-Farber; Editing by Alison Williams, Alexandra Hudson)