The former US envoy who flew to North Korea to secure the release of an American detainee in 2017 said Monday that he believes the Trump administration should honor its pledge to pay $2 million to the regime.

Joseph Yun, then the US special representative for North Korea policy, made the remark in an interview on CNN, after confirming that he signed a letter of assurance on the payment before he brought home American college student Otto Warmbier.

Warmbier fell into a coma shortly after he was detained in Pyongyang in early 2016, and the North Koreans had demanded the sum as payment for his medical expenses.

The 22-year-old died several days after he was released and flown back to the US in June 2017.

"As soon as the North Korean side told me that his bill for $2 million would have to be paid, of course, I contacted my boss, then Secretary of State Rex Tillerson, to ask him," Yun said. "And he got back to me very quickly thereafter to say, 'Yes, go ahead and sign.'"

The former envoy said it was his understanding that Tillerson had instructed him to do so with the approval of President Donald Trump, although he never asked.

The Washington Post first reported on the bill last week and Trump has since denied that any money was paid to the North.

"I know until I had left, US government had not paid the money," said Yun, who retired from the State Department in March last year.

"Having signed, should we pay? My view is yes," he continued.

"If you've signed, if you've promised another government from the US government that you would make the payment, my view certainly is that we should go ahead and meet our end of the commitment." (Yonhap)