Otto Frederick Warmbier, a University of Virginia student who has been detained in North Korea since early January, attends a news conference in Pyongyang, North Korea, in this photo released by Kyodo February 29, 2016.

The former U.S. diplomat who secured the release from North Korea of American Otto Warmbier said on Monday Washington should honor its pledge to pay Pyongyang $2 million for the student's hospital care.

Joseph Yun, the former U.S. special envoy for North Korea, confirmed to Reuters he had signed an agreement with the North Koreans for the payment of the money before Warmbier was flown out of Pyongyang in a coma on June 13, 2017.

He died days after returning home.

White House national security adviser John Bolton said in a television interview on Sunday that while the agreement had been signed, no payment had been made to the North Koreans.

Yun, who retired from the State Department last year and is now a global affairs analyst for CNN, said on the network he believed the United States should honor its commitment.

"My view is, yes. If you've signed, if you promised another government from the U.S. government that you would make the payment, my view certainly is that we should go ahead and meet our end of the commitment," he said.

Yun said that as soon as North Korea made the demand he contacted the then secretary of state, Rex Tillerson, who told him to go ahead and sign the agreement. Asked if he believed Tillerson had the approval of President Donald Trump, he replied: "That was my understanding. I never asked him. But that was my understanding."

The Washington Post reported last Thursday that Trump had approved payment of the $2 million bill from Pyongyang to cover its care of Warmbier, who was detained in North Korea for 17 months.

In a reply on Twitter, Trump said "no money was paid" but did not address whether any agreement had been made.

Warmbier died in the United States six days after his release from North Korea. An Ohio coroner said he died from a lack of oxygen and blood to his brain. North Korea has dismissed claims that it tortured the student and blamed food poisoning and a sleeping pill.

North Korea state media said Warmbier, a University of Virginia student visiting North Korea, was sentenced to 15 years hard labor for trying to steal an item bearing a propaganda slogan from his hotel.