"I would say it was a productive set of discussions over the last few days, and our team engaged on a number of areas of mutual interest, and we've agreed to meet again," he said during a meeting with his South Korean counterpart Lee Do-hoon.

"So I think this is a constructive place to be especially in advance of the president's second summit with Chairman Kim," he added.

During a courtesy call on Foreign Minister Kang Kyung-wha earlier in the day, Biegun cited "some hard work" to do with the North before the Trump-Kim summit but did not elaborate.

"We don't know where it is going to go, but we are in the midst of a conversation with the North," he said.

"I am confident that both sides stay committed, that we can make real progress," he added.

Giving Seoul's "full" support for summit preparations, Kang noted that the past week has been a "long and momentous" week for the US negotiator.

"I think you've come back with outcomes from Pyongyang that we can both build upon, first of all for the very successful second summit between President Trump and Chairman Kim," she said.

Biegun credited Kang and South Korean President Moon Jae-in for "having created the opening that has allowed much of this to take place."

The US envoy dropped by Seoul to give South Korean and Japanese officials briefings on the results of his high-stakes visit to the North Korean capital.

He is set to hold a trilateral luncheon meeting with Lee and Kenji Kanasugi, the Japanese foreign ministry's director general of Asia and Oceania affairs.

In Pyongyang, Biegun met his North Korean counterpart Kim Hyok-chol, a former ambassador to Spain, with an aim to make progress toward the "complete denuclearization" of the Korean peninsula that their leaders agreed to in their first historic summit in Singapore in July.

Under the vision of "final, fully" verified denuclearization, Washington has been pressuring Pyongyang to take concrete, substantive steps for its nuclear disarmament, including a declaration of its nuclear and missile programs.

The North has demanded the United States' corresponding measures for its denuclearization steps that may include partial sanctions relief, establishment of a US liaison office in Pyongyang and a declaration of a formal end to the 1950-53 Korean War. (Yonhap)