Secretary of State Colin Powell was asked about the claim, made public by US officials insisting on anonymity last week, after talks here with South Korean Foreign Minister Yoon Young-Kwan.
"That's what they said, I don't know if it was a promise or just a statement," said Powell, in the first on the record comment on the alleged threat by a senior US official.
Yoon and Powell both went to the White House to meet President George W. Bush to discuss how to move on from the inconclusive Beijing talks, which ended with an undertaking to meet again, but with no date set.
"He had a very, very good conversation with President Bush for a few moments before we came back here for conversations with the minister and his delegation," Powell told reporters.
Yoon was also due to meet Bush's national security advisor Condoleezza Rice and Deputy Secretary of State Paul Wolfowitz during his visit which ends on Friday, South Korean officials said.
US claims that North Korea had threatened to test a nuclear device, and to show it had the means to deliver one, cast a cloud over the Beijing talks.
But Russia's top negotiator on North Korea, Deputy Foreign Minister Alexander Losyukov, denied Friday that Pyongyang had given any such warning.
Powell said despite what he called North Korean threats, Washington would "stay the course" and look for a diplomatic way out of the nuclear showdown which erupted in October.
But he said that process was "not helped by threats and truculent statements that are designed to try to frighten the international community and try to frighten us."
"We will not be frightened nor will we be caused by such threats to take actions that we do not believe are in our interests or in the interests of our partners.
"We hope that the North Koreans will understand the seriousness of our position, our proposals and will respond in a serious way and not with these kinds of rhetorical threats."
In a bombastic statement over the weekend, North Korea described the six-party crisis talks also involving the United States, South Korea, Japan, China and Russia as "useless" adding that it intended to build up its nuclear deterrent.
But Tuesday, an apparently contradictory statement from the North's official news agency said the Stalinist state wanted to resolve the issue "peacefully, through dialogue."
The commentary was seen as confirmation by the communist state it plans to press on with negotiations.
Powell was also asked about a highly critical appraisal of US policy towards North Korea in Manila by Chinese Vice Foreign Minister Wang Yi, who said Washington was the "main problem" blocking a solution to the crisis.
But he said no one in Washington, or Chinese officials contacted by the State Department, had been able to unearth a transcript of what Wang, who was quoted in a sheaf of news reports, actually said.
"But I think the context of his comment is that both sides have to show openness and flexibility," Powell said.
"I am quite sure the vice foreign minister was not resting the problem on the United States.
"He understands, as do all of the other members who were in the room that day, that the fundamental problem is North Korea nuclear weapons development not the United States policies."
Regarding the timing of a future round of the multilateral talks, which the United States sees as the only way out of the crisis, Powell said it was too soon to say.
"Our Chinese colleagues who are serving as the host of the talks and convened them last time certainly I think are anticipating that we would have another round in the not-too-distant future.
"But it remains to be seen. It takes six people to agree to talks to have talks and we'll see what unfolds over the next several weeks."