North Korea has pledged to destroy all its nuclear material enrichment facilities, according to the US special envoy for the country, Stephen Biegun.
He said the promise was made to US Secretary of State Mike Pompeo when he visited North Korea in October.
But Mr Biegun said North Korea must provide a complete list of its nuclear assets before any deal can be reached.
President Donald Trump had earlier claimed “tremendous progress” in talks between the countries.
Speaking in the Oval Office on Thursday, the president said he would soon announce the date and location of a planned second summit with North Korea’s Kim Jong-un.
The pair met in Singapore last year, the first meeting between a sitting US president and a North Korean leader, and signed an optimistic but vague declaration of their commitment to denuclearisation.
Since then little progress has been made.
North Korea has always said its nuclear programme is essential to its survival and that it will never unilaterally give it up unless it no longer faces a nuclear threat from the US.
In his speech at Stanford University in California, Mr. Biegun said President Trump was “ready to end this war”.
“We’re not going to invade North Korea. We are not seeking to topple the regime,” said Mr Biegun, who stepped into the role five months ago.
He said Kim Jong-un had committed, in his talks with Mr Pompeo, to “the dismantlement and destruction” of all its plutonium and uranium facilities, which provide the material for nuclear weapons.
But he reiterated that the US would not lift sanctions until denuclearisation was complete, demanding “a complete understanding of the full extent of the North Korean WMD [weapons of mass destruction] and missile programmes through a comprehensive declaration”.
North Korea has long refused to give a full account of its nuclear capacity, and the means by which any surrendering or dismantling of nuclear arms will be verified has always been a sticking point in negotiations.
Mr. Biegun said the US position remained that it would not lift sanctions “until denuclearisation is complete”, but indicated it could provide assistance in other ways, saying: “We did not say we will not do anything until you do everything.”
He also said there had been no discussion with North Korea on whether the 28,500 US military personnel stationed in South Korea could be withdrawn as a concessional move.
Mr. Biegun, who is due to meet his North Korean counterpart in South Korea on 3 February, said the US had “contingencies” in place if the diplomatic process collapsed.
Despite positive noises from the White House, there are still sceptics on the Korean peninsula and in the US.
US intelligence chiefs presented the annual Worldwide Threat Assessment report to the US Senate earlier this week, which assessed that North Korea is “unlikely to give up” its weapons programme.
The report also said Iran was not developing nuclear weapons, as the Trump administration has said, prompting a tweet from the president telling the intelligence heads to “go back to school”.
A further tweet on Thursday claimed the Senate report was “mischaracterised by the media”.
Just concluded a great meeting with my Intel team in the Oval Office who told me that what they said on Tuesday at the Senate Hearing was mischaracterized by the media – and we are very much in agreement on Iran, ISIS, North Korea, etc. Their testimony was distorted press…. pic.twitter.com/Zl5aqBmpjF
— Donald J. Trump (@realDonaldTrump) January 31, 2019