Rafah's population has swelled from 250,000 to 1.5 million as Palestinians have fled to the territory's southernmost city
A top UN official has warned an Israeli assault on Rafah, Gaza's southernmost city, could lead to a "slaughter".
Humanitarian chief Martin Griffiths said Palestinians in Gaza were already suffering an "assault that is unparalleled in its intensity, brutality and scope".
The consequences of an invasion of Rafah would be "catastrophic", he said.
Israeli Prime Minister Benjamin Netanyahu has vowed to defeat Hamas gunmen he says are hiding in the city.
In an unusually strongly worded statement, Mr Griffiths said over a million people were "crammed in Rafah, staring death in the face". He said civilians in the city had little food or access to medicine and "nowhere safe to go".
An Israeli invasion of the city, he added, would "leave an already fragile humanitarian operation at death's door".
A spokesperson for UN Secretary General Antonio Guterres told the BBC's Newshour programme the UN had not received any Rafah evacuation plans from Israel and would not participate in any forced evacuation.
Stephane Dujarric said: "The United Nations will not be party to any forced displacement of people."
Rafah is a small city in the south of the Gaza Strip on the border with Egypt. Before the war it was home to around 250,000 people, but since Israel ordered civilians to evacuate south its population has swelled to an estimated 1.5 million.
Many are living in tents in desperate conditions and say they have nowhere to go.
Rafah has come under heavy Israeli air strikes in recent days, with at least 67 people killed there on Monday according to the Hamas-run health ministry.
Watch: How did Rafah become home to 1.5 million Palestinians?
Mr Griffiths also said humanitarian workers working in Gaza had been "shot at, held at gunpoint, attacked and killed" because of the breakdown in law and order.
His statement came as negotiations for a ceasefire between Israel and Hamas in Gaza resumed in Cairo.
Senior officials from the US, Israel, Egypt and Qatar met on Tuesday, as pressure mounted on Israel from the international community not to invade Rafah.
Mr Guterres said that hoped the talks would be successful so as to avoid an Israeli attack on the city.
US President Joe Biden has warned Israel that civilians must be protected. UK Foreign Secretary David Cameron has told Israel to "stop and think seriously" before attacking Rafah.
At least 1,200 people were killed during attacks in Israel by Hamas-led gunmen on 7 October last year.
In response, Israel launched a military campaign in the Gaza Strip. More than 28,400 Palestinians, mostly women and children, have been killed and more than 68,000 wounded since the war began, according to the Hamas-run health ministry.