The Israeli military did not immediately provide comment on the warning, which came as Israel amassed tanks near the Gaza border and pounded the Palestinian enclave with air strikes following a deadly Hamas attack in Israel.

“The UN considers it impossible for such a movement to take place without devastating humanitarian consequences,” UN spokesman Stephane Dujarric said in a statement.

“The UN strongly appeals for any such order, if confirmed, to be rescinded avoiding what could transform what is already a tragedy into a calamitous situation,” he said.

Dujarric said the order by the Israeli military also applied to all UN staff and those sheltered in UN facilities, including schools, health centres and clinics.

Israel’s mission to the UN in New York did not immediately respond to a request for comment.

Hamas’ weekend rampage killed 1,300 people and was the worst attack on civilians in Israel’s history.

Israel has responded so far by putting Gaza, home to 2.3 million people, under siege and launching a bombing campaign that destroyed whole neighbourhoods. Gaza authorities said more than 1,500 Palestinians have been killed.