JERUSALEM — In the Khan Younis refugee camp in Gaza, Mahmoud Ferwana, 59, huddled beneath a flimsy nylon-and-sheet-metal roof while rain drenched the rest of his squalid home’s sandy floor. He earns money collecting broken stones; his children scavenge for copper. But none of it amounts to a living.

The United Nations Relief and Works Agency is what reliably puts food in their mouths.

If that aid were to stop, Mr. Ferwana said on Wednesday, “I and my children will die.”

That aid, to Mr. Ferwana and more than five million other Palestinians living in refugee camps across the Middle East, is now endangered by what the agency’s leaders are calling the worst financial crisis in its seven-decade history.

The United States, its biggest donor, announced this week that it was withholding $65 million from a scheduled payment of $120 million. The Trump administration said it was pressing for unspecified reforms from the agency, while also seeking to get Arab countries to contribute more.