The US Secretary of State Mike Pompeo arrived in North Korea on Friday, hoping to "fill in" details on the North's plans to dismantle its nuclear programme and also to secure the remains of US troops missing from the Korean War.
Mr Pompeo met Kim Yong Chol, a senior North Korean official who played a central role with the secretary of state in arranging last month's summit between President Donald Trump and North Korean leader Kim Jong-un in Singapore.
Pompeo will stay in North Korea until Saturday, his first overnight stay in the country.
At the Singapore summit, Mr Kim made a broad commitment to "work toward denuclearisation", but fell short of details on how or when he would dismantle North Korea's nuclear programmes.
"The president told me he believes that Chairman Kim sees a different, brighter future for the people of North Korea. We both hope that's true," Mr Pompeo tweeted after a phone call with Mr Trump during his flight.
"Next stop: Pyongyang. I look forward to continuing my meetings with North Korean leaders. There's much hard work ahead but peace is worth the effort."
Mr Pompeo said he was seeking to "fill in" some details on North Korea's commitments and maintain the momentum toward implementation of the agreement from the summit.
Mr Pompeo would try to agree on at least an initial list of nuclear sites and an inventory that could be checked against the available intelligence, US intelligence officials told Reuters.
Also high on the agenda is the issue of the remains in North Korea of US soldiers missing from the 1950-53 war. Mr Trump said after the Singapore summit that Mr Kim had agreed to send remains back to the United States.
Both issues are considered essential tests of whether Mr Kim is serious about negotiations. North Korean officials have yet to demonstrate that in working-level talks, the intelligence officials said.
"If they're serious, then we can get down to the business of defining the terms of final denuclearisation," said one official.
But the US ability to verify the accuracy of any North Korean list is limited due to the lack of a "high confidence" accounting of the North's nuclear arsenal, such as the number of warheads and uranium enrichment facilities, especially if they are not operational, they said.
And while the Pentagon has said in the past that North Korean officials have indicated they had the remains of as many as 200 US troops, a US military official familiar with the procedures for handling remains said it was not clear what North Korea might hand over.
"Until we do the necessary DNA testing to verify whose remains they are, and things like whether they've put remains of the same soldier into more than one box or tried to fool us with pieces of animal bones, we won't know for sure what they've given us back," the official said.
Some officials in the State and Defence Departments and in US intelligence agencies are worried that by overstating the results of the Singapore summit, Mr Trump has put himself at a disadvantage if negotiations do begin.
Ahead of that meeting, Mr Pompeo had said Mr Trump would reject anything short of "complete, verifiable and irreversible denuclearisation".
But following talks on Sunday between US envoy Sung Kim and North Korean counterparts, this "CVID" language appears to have disappeared from the State Department lexicon.
It says pressure will remain until North Korea denuclearises, but in statements this week, it redefined the US goal as "the final, fully verified denuclearisation" of the country.
Some US officials and experts have said the change in language amounted to a softening in the US approach.
The State Department denied the view, saying its policy remains unchanged.
After his departure from Washington on Thursday, Mr Pompeo tweeted: "Looking forward to continuing our work toward the final, fully verified denuclearization of #DPRK, as agreed to by Chairman Kim. Good to have press along for the trip."
"The president has made it hard to walk away from the talks even if the North is just stalling and prevaricating again," said another US official familiar with the talks, pointing to Mr Trump's tweets that North Korea no longer poses nuclear threats.
"Kim may be betting – maybe gambling – that just as he agreed to meet after threatening fire and fury, the president may back down again and let Pyongyang set the agenda and the timetable," the official said.
Mr Pompeo's talks will be closely watched in the region. He is due to meet officials from allies South Korea and Japan in Tokyo on Sunday.
A spokesman for South Korea's presidential office would only say South Korea and the United States were working to formulate "constructive measures" on North Korea's denuclearisation.