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 had been made to US Secretary of State Mike Pompeo when he visited North Korea in October.
However Pyongyang has not confirmed making any such pledge.
Mr Biegun also said that North Korea must provide a complete list of its nuclear assets before any deal can be reached.
Experts believe the North has more than one undeclared nuclear fuel enrichment site aside from the known facility at Yongbyon, north of Pyongyang, and question how the destruction of all facilities could be fully verified.
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.
Second Trump-Kim meeting ‘within weeks’
When Trump met Kim: What happened next?
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.
What have we learned?
Stephen Biegun has been Washington’s top envoy to North Korea for five months but he gave a detailed public accounting of his approach for the first time in a 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,” he said.
Mr Biegun – who will travel to South Korea on Sunday before meeting North Korean officials – also said Washington was willing to discuss a range of trust-building measures with Pyongyang.
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”.
Is North Korea sticking with its nukes?
North Korea’s breakdown of communication
What do we know about these facilities?
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.
The country has one known nuclear fuel production facility at Yongbyon, which has been the source of plutonium for its nuclear weapons programme. Uranium is also enriched there, according to experts.
However there is believed to be at least one other undeclared facility to enrich nuclear fuel in North Korea.
North Korea’s missile and nuclear programme
US intelligence officials believe Pyongyang has stepped up the enrichment of uranium even while it continues talks with Washington, according to reports.
So far Kim Jong-un has committed to allowing experts to verify the destruction of two sites – the Tongchang-ri missile engine testing site and the Punggye-ri nuclear test site.
But Mr Biegun admitted that these facilities are “not critical parts of the current North Korean missile or nuclear programmes”.
North Korea carried out a series of long-range missile tests in 2017
What else did Biegun say?
He 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 admitted that North Korea and the US did not have a shared definition of what denuclearisation actually meant.
“We do not have a specific and agreed definition of what final, fully verified denuclearisation or comprehensive, verifiable, irreversible denuclearisation – whatever your preferred term of art – is,” he said.
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 said the US had “contingencies” in place if the diplomatic process collapsed.
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.
Kim Jong-un, left, has met South Korea’s Moon Jae-in several times
What do others think?
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 said the Senate report had been “mischaracterised by the media”.