National security adviser John Bolton John BoltonMaximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales Ex-NSC official alleges 'unprecedented' intervention by White House aides in Bolton book review MORE said Sunday that the U.S. "absolutely" did not pay North Korea for the release of Otto Warmbier, but confirmed that a U.S. envoy did sign a pledge agreeing to make a payment.

“Absolutely not. And I think that’s the key point. The president’s been very successful in getting 20-plus hostages released from imprisonment around the world and hasn’t paid anything for any of them," Bolton said during an interview on "Fox News Sunday."

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Bolton added, however, that Joseph Yun, a State Department envoy sent to retrieve Warmbier from North Korea in 2017, signed a pledge to pay the country for Warmbier's release.

“That is what I am told, yes," Bolton said.

Bolton's remarks come after The Washington Post reported last week that North Korea issued a $2 million bill to the U.S. for the hospital care of Warmbier and that Yun signed a pledge saying the U.S. would pay the bill. The Post reported that it was unclear whether the bill was ultimately paid.

President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Friday also denied that the U.S. paid any money to North Korea for Warmbier.

"No money was paid to North Korea for Otto Warmbier, not two Million Dollars, not anything else," he tweeted.

North Korea in 2016 sentenced Otto Warmbier to 15 years of hard labor, and the former University of Virginia student fell into a coma shortly thereafter. Warmbier was still in a coma when he was returned to the U.S. in 2017 and died without regaining consciousness.