The US special representative for North Korea is travelling to Vietnam to continue preparations for next week's second summit between US President Donald Trump and North Korean leader Kim Jong-Un.
State Department spokesman Robert Palladino told a news briefing he had no details of the meetings Stephen Biegun would have in Hanoi.
Biegun spent three days in North Korea from February 6-8, a trip he said was aimed at agreeing on "concrete deliverables" for the February 27-28 summit.
Biegun said after that trip his talks in North Korea had been "productive" but there was "hard work to do" before the summit.
The United States has been demanding that North Korea give up a nuclear weapons program, and Trump has been eager for a second summit even though a first meeting in Singapore in June meeting produced only vague commitments.
North Korea has been seeking a lifting of punishing US-led sanctions, a formal end to the 1950-53 Korean War and security guarantees.
Asked whether the United States would consider lifting sanctions, Palladino said: "We've been clear on sanctions. These are the world's sanctions and that is something that ... will continue to be maintained until we've achieved our final result of a fully, finally, verified denuclearization."
Palladino did not respond directly when asked about reports that the two sides were discussing the exchange of liaison officers.
US Secretary of State Mike Pompeo said of Kim last week it was "time for him to deliver," but the Trump administration has moved away from demands that North Korea give up its nuclear weapons immediately and has appeared to adopt a more gradual, reciprocal approach.
On Tuesday, Trump reiterated that he wants North Korea to end its nuclear program, but he is in no rush and has no pressing time schedule.
