In a statement, the State Department said talks during Stephen Biegun's three-day trip explored Trump and Kim's "commitments of complete denuclearization, transforming US-DPRK relations and building a lasting peace on the Korean Peninsula" in preparation for the much-anticipated summit in Vietnam on February 27 and 28.

Biegun landed at Osan US Air Base Friday evening, foreign ministry spokesman Noh Kyu-duk told AFP.

The State Department confirmed Biegun agreed to meet his North Korean counterpart Kim Hyok Chol again before the summit.

Biegun is expected to share details of his Pyongyang meetings with his South Korean counterpart Lee Do-hoon and Foreign Minister Kang Kyung-wha on Saturday.

Attention will focus on whether the US team have offered to lift some economic sanctions in return for Pyongyang taking concrete steps towards denuclearisation.

Discussions on declaring an end to the 1950-53 Korean War could also have been on the table, with Biegun last week saying Trump was "ready to end this war."

The three-year conflict ended in an armistice, not a peace treaty, leaving the two Koreas still technically at war, with the US keeping 28,500 troops in the South.

The US envoy was also likely to have discussed with his counterpart protocol and security matters for the upcoming Trump-Kim summit.

At their landmark summit in Singapore last year, the mercurial US and North Korean leaders produced a vaguely-worded document in which Kim pledged to work towards "the denuclearization of the Korean peninsula."

But progress has since stalled, with the two sides disagreeing over what that means.

Experts say tangible progress on Pyongyang's nuclear weapons will be needed for the second summit if it is to avoid being dismissed as "reality TV."