(Reuters) -Israel's military informed the United Nations early on Friday that the 1.1 million Palestinians in Gaza should relocate to the enclave's south within the next 24 hours, a U.N spokesman said, in what Palestinians fear could be a precursor to a planned Israeli ground offensive.

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 militant attack in Israel.

"The United Nations considers it impossible for such a movement to take place without devastating humanitarian consequences," U.N. spokesman Stephane Dujarric said in a statement.

"The United Nations strongly appeals for any such order, if confirmed, to be rescinded avoiding what could transform what is already a tragedy into a calamitous situation," he said.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 U.N. staff and those sheltered in U.N. facilities, including schools, health centres and clinics.

Israel's mission to the United Nations 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.

(Reporting by Michelle Nichols; Writing by Rami Ayyub; Editing by Raju Gopalakrishnan)