“We don’t have what we need,” Mr. Halabi said. “We can barely find water.”

More than 800,000 people have left Rafah in the past two weeks, a United Nations official said Monday. The Israeli military said the same day that more than 950,000 of the city’s civilians had moved since it gave expanded evacuation orders. A military spokesman said between 300,000 and 400,000 civilians remain there.

The latest wave of displacement in Gaza began on May 6, when Israel sent evacuation warnings and launched military operations in eastern Rafah, along the border with Egypt. More than half of the enclave’s civilians had sought refuge in the city, most of them after fleeing fighting elsewhere in Gaza several times.

Ali Jebril, 27, a wheelchair-bound basketball player, said he and his family paid $600 to take 35 people from Rafah east to Khan Younis by bus earlier this month.

Mr Jebril, who said his wheelchair cannot navigate the sandy beach areas where many have resettled, moved to a tent on the grounds of a hospital in Khan Younis.

“We are not living a dignified life,” he said. “We are facing a catastrophe.”

The war, he said, made him feel like he had become a burden on society, often asking others to help him.

Since Israel’s incursions into Rafah, the city’s once overcrowded shelters and tent cities have largely emptied, Edem Wosornu, an official at the United Nations office for humanitarian affairs, told the Security Council on Monday. People have moved to areas near Khan Younis and Deir al Balah and set up makeshift camps without toilets, water, drainage or shelter, he said.

“We described it as a catastrophe, a nightmare, like hell on earth,” Ms. Wosornu said. “It’s all that, and worse.”

Since the war began in October, three-quarters of Gaza’s population has been displaced, and many people have moved four or five times, he said.

Israel interpreted the orders as a humanitarian step to protect civilians ahead of further military action, which it says is necessary to root out Hamas fighters in southern Gaza. But aid groups say the additional displacement is worsening an already catastrophic humanitarian situation.

In its latest update, the United Nations Office for the Coordination of Humanitarian Affairs described people living in clusters of 500 to 700 tents, many of them made of blankets, nylon or whatever other materials are available. According to the report, some tents have been set up on an unstable slope of the beach, with rubbish from higher areas rolling downstream past homes into the sea.

Mr Halabi said food was available in markets, but that his family was so short of money that paying for it was difficult.

“After seven months of war, we have almost nothing,” he said.

While an increasing number of commercial trucks have entered Gaza recently, aid reaching the south through the Kerem Shalom and Rafah crossings has all but stopped. UNRWA, the main U.N. agency for aid to Palestinians, said that in a 16-day period through Tuesday, only 69 humanitarian trucks entered through the two crossings – the lowest rate since the first weeks of the war.

Philippe Lazzarini, head of the main U.N. agency helping the Palestinians, wrote in a social media post that any transfer carries risks and takes a heavy toll.

“Each time they are forced to leave behind the few belongings they have: mattresses, tents, kitchen utensils and basic necessities that they cannot transport or pay for transport,” he wrote. “Every time they have to start over, all over again. “