U.S. Special Representative for North Korea Stephen Biegun said Saturday that his three-day talks in Pyongyang were "productive."

During a meeting with South Korean Foreign Minister Kang Kyung-wha in Seoul, the U.S. envoy said "we don't know where it is going to go" but the U.S. is in the midst of a conversation with the North.

Biegun cited "some hard work" to do with the North but said he is confident that both sides stay committed and real progress can be made. 

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

Kang earlier mentioned the first year anniversary of the PyeongChang Winter Olympics. 

She also thanked the U.S. negotiator for visiting Seoul before returning to Washington to share the results of his trip to Pyongyang.

In a subsequent meeting with Seoul's top nuclear envoy Lee Do-hoon, Biegun said that it was a productive set of discussions over the last few days and his team engaged on a number of areas of mutual interest and the U.S. and North Korea also agreed to meet again.

He said this is a constructive place to be, especially in advance of President Trump's second summit with Chairman Kim Jong-un.

In response, Lee said Seoul is ready to discuss ways to make further progress.

Lee and Biegun also held a trilateral luncheon meeting with Kenji Kanasugi, the Japanese Foreign Ministry's director general of Asia and Oceania affairs.

Biegun flew back to Seoul on Friday evening following a three-day visit to Pyongyang aimed at fine-tuning preparations for the second North Korea-U.S. summit scheduled for February 27th and the 28th in Vietnam. His North Korean counterpart for the talks was Kim Hyok-chol, Pyongyang's former ambassador to Spain. 

Trump earlier tweeted that the summit will take place in Hanoi.