More than a million people in Rafah are "staring death in the face" as an Israeli assault looms, a senior United Nations official has warned.
UN humanitarian chief Martin Griffiths said a strike on Gaza's southernmost city could lead to a "slaughter".
Palestinians in the besieged enclave are already suffering an "assault that is unparalleled in its intensity, brutality and scope" and the consequences of an invasion of Rafah would be "catastrophic", he added.
In an unusually firm statement, Mr Griffiths said civilians in Rafah had little access to food or medicine and an invasion of the city would "leave an already fragile humanitarian operation at death's door".
Rafah, which had a pre-war population of about 300,000, teems with homeless people living in tent camps and makeshift shelters who have fled from bombardments in other areas of the territory during more than four months of war.
It comes as Gaza truce talks involving the US, Egypt, Israel and Qatar ended without a breakthrough on Tuesday.
Israeli Prime Minister Benjamin Netanyahu has argued Hamas fighters are hiding in the city and the country must defeat them.
The Tel Aviv Government says it wants to flush out Hamas militants from hideouts in Rafah and free Israeli hostages being held there.
Its military is making plans to evacuate Palestinian civilians. But no plan has been forthcoming and aid agencies say the displaced have nowhere else to go in the shattered territory.
Israeli tanks shelled the eastern sector of Rafah overnight, causing waves of panic, residents said.
A spokesperson for UN Secretary General Antonio Guterres told the BBC that the UN had not received any Rafah evacuation plans from Israel and would not participate in "any forced displacement of people".
