But in the 1990s, as Cambodia began to emerge from decades of war, the United States said the money was still owed, with interest and late fees, though it offered rescheduling on favorable terms. Since then the debt has swelled to $506 million.

“We lack the legal authority to write off debts for countries that are able but unwilling to pay,” Jay Raman, a spokesman for the United States Embassy in Phnom Penh, said in an email last month. “These legal authorities do not change from one administration to the next, absent an action from Congress.”

Cambodia argues that the loan is invalid because the government of Lon Nol, who seized power in a 1970 coup that deposed Prince Norodom Sihanouk, was illegitimate. But the State Department says the international financial system will fall apart if governments cannot be held responsible for their predecessors’ debts.

The United States has also disputed arguments by Cambodia that it cannot afford to repay the debt. Once one of the world’s very poorest countries, Cambodia graduated to lower-middle income status last year, with a gross domestic product of about $19 billion, according to the International Monetary Fund. Refusing to service the American loan has impeded its ability to borrow internationally.

“I look around me, and to me Cambodia does not look like a country that should be in arrears,” the American ambassador, William Heidt, told local journalists in February. He said that the United States wanted to “work out a deal that works for both sides” but that completely canceling the debt was not an option.

“From time to time, for reasons I don’t think that we really fully understand, the Cambodian government feels the need to publicly criticize the United States,” Mr. Heidt said. “I think that reflects some kind of political dynamic inside of Cambodia.”