By Charles Scanlon
BBC correspondent in Seoul
A senior US official says Washington's commitment to peace may be undermined if North Korea continues to deny part of its nuclear weapons capabilities.
John Bolton is in the region ahead of talks in Beijing
Under-Secretary of State John Bolton was speaking a week before negotiations on the issue resume in Beijing.
"If they truly want to get rid of their nuclear weapons programme, here is the opportunity to do it," he said.
The US is urging North Korea to follow the example of Libya and give up all its nuclear facilities.
United States officials have been talking tough in the run-up to the new round of talks with North Korea.
Now John Bolton, well known for his hardline views, has given the strongest warning so far.
He said the US commitment to a peaceful solution could be subverted if North Korea continued to deny a key part of its atomic bomb programme.
The North claims to have extracted enough plutonium for several atomic bombs over the last year.
It has offered to freeze its facilities in return for economic aid from the United States.
However, it strongly denies American allegations that it is also running an entirely separate secret programme based on the enrichment of uranium.
Mr Bolton described the uranium programme as an 800lb (298 kg) gorilla sitting at the table that could not be ignored.
He said the recent public confession by the Pakistani nuclear scientist, AQ Khan, had backed up American intelligence about North Korea's capabilities.
American officials say they will not submit to blackmail or reward bad behaviour.
They are demanding the total and irreversible scrapping of all North Korea's nuclear capabilities.
President Bush has said repeatedly that he wants to solve the problem through regional diplomacy, but some in the administration are reported to favour tougher measures including economic sanctions.