CAIRO, Feb 16 (Reuters) - Egypt is preparing an area at the Gaza border which could accommodate Palestinians in case an Israeli offensive into Rafah prompts an exodus across the frontier, four sources said, in what they described as a contingency move by Cairo.

Egypt, which denied making any such preparations, has repeatedly raised the alarm over the possibility that Israel's devastating Gaza offensive could displace Palestinians into Sinai - something Cairo says would be completely unacceptable- echoing warnings from Arab states such as Jordan.

The United States has repeatedly said it would oppose any displacement of Palestinians out of Gaza.

One of the sources said Egypt was optimistic talks to clinch a ceasefire can avoid any such scenario, but is establishing the area at the border as a temporary and precautionary measure.

Three security sources said Egypt had begun preparing a desert area with some basic facilities which could be used to shelter Palestinians, emphasising this was a contingency step.

The sources Reuters spoke to for this story declined to be named because of the sensitivity of the matter.

Israel has said it will mount an offensive to take out Hamas's "last bastion" in Rafah, where well over 1 million Palestinians have sought sanctuary from its devastating Gaza offensive.

Israel has said its army is drawing up a plan to evacuate civilians from Rafah to other parts of the Gaza Strip.

But U.N. aid chief Martin Griffiths said on Thursday it was an "illusion" to think people in Gaza could evacuate to a safe place and warned of the possibility of Palestinians spilling into Egypt if Israel launches a military operation in Rafah.

He called this scenario "a sort of Egyptian nightmare".

Egypt has framed its opposition to the displacement of Palestinians from Gaza as part of wider Arab rejection of any repeat of the "Nakba", or "catastrophe", when some 700,000 Palestinians fled or were forced from their homes in the war surrounding Israel's creation in 1948.

The first source said construction of the camp began three or four days ago and it would offer temporary shelter in any scenario of people crossing the frontier "until a resolution is reached".

Asked about the accounts by the sources, the head of Egypt's State Information Service said: "This has no basis in truth. Our Palestinian brothers have said and Egypt has said that there is no preparation for this possibility."

Sinai Foundation for Human Rights, an activist organisation, published images on Monday it said showed construction trucks and cranes working in the area and images of concrete barriers.

Citing an unidentified source, the Sinai Foundation said that the construction work was intended to create a secured area in case of a mass exodus of Palestinians.

Photo from Reuters