A recurring demand by the United States that Cambodia repay $500m in "war debt" has prompted fury in the Southeast Asian nation.

The loan started out as $274m in the 1970s, which the US says was used for food supplies, but Cambodian Prime Minister Hun Sen has called it "dirty money" and said it was used to buy weapons.

Sen said the US had no right to demand repayment of a debt that was "blood-stained" from US bombing of Cambodian territory during the Vietnam war.

"The US created problems in my country and is demanding money from us," he said, according to local media. "We also don't demand that the US pay for the damage and destruction caused by the war. We just want the US to be responsible for the problem of the debt."

Sen called on US President Donald Trump to cancel the debt late last year, but Washington ignored pleas to cancel the decades-old arrangement.

READ MORE: Cambodians mark 40 years since Khmer Rouge takeover

Between 1972 and 1974, the US Department of Agriculture financed $274m in purchases of US cotton, rice and flour by the US-backed Khmer Republic, then an ally in the war to stem the spread of communism in Southeast Asia.

During that time, the US dropped more than 500,000 tonnes of explosives on Cambodia's countryside.

Journalist Elizabeth Becker, who covered the Cambodian genocide in the 1970s, told Al Jazeera it was immoral for the US to ask for repayment.

"US would not drop it. It would have been so easy to forgive the repayment, it would have been easy to refinance it for education like they did in Vietnam," she said.

"The US intervention in Cambodia was easily the most controversial that we had in that era. They [US] dragged Cambodia into the Vietnam War for hopes that by expanding it they could win, the complications now are that even 50 years later, the Khmer Rouge legacy is horrible."