Kim Hyok Chol arrived in the Chinese capital at around 10 am (0200 GMT) and was expected to board a plane bound for Hanoi later in the day.

His trip comes three days after Kim Jong Un's de-facto chief of staff, Kim Chang Son, landed in Hanoi to discuss protocol and security matters with the US team ahead of the summit on February 27-28.

Kim Hyok Chol and his US counterpart Stephen Biegun were engaged in three days of talks in Pyongyang earlier this month, exploring each side's positions on denuclearisation ahead of the much-anticipated meeting.

Biegun said they had been productive, but more dialogue was needed.

"We have some hard work to do with the DPRK between now and then," Biegun said, adding that he was "confident that if both sides stay committed we can make real progress here".

The US State Department said talks during Biegun's trip explored Trump and Kim Jong Un's "commitments of complete denuclearisation, transforming US-DPRK relations and building a lasting peace on the Korean Peninsula".

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

Alex Wong, US deputy assistant secretary of state for North Korea, is already in the Vietnamese capital preparing for the summit.

Biegun is expected to fly there soon from Washington to resume talks with Kim Hyok Chol.

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".