Secretary-General Antonio Guterres warned Thursday that negotiations between the two Koreas won't be successful unless there is simultaneous success in U.S.-North Korean negotiations to denuclearize the Korean Peninsula.

The U.N. chief told a news conference that the results of this week's summit between South Korean President Moon Jae-in and North Korean leader Kim Jong Un "were positive," but progress is also essential between Washington and Pyongyang.

"It is clear for me that there will not be success in inter-Korean negotiations if simultaneously there is not success in the American-North Korea ... negotiations to achieve the full and verifiable denuclearization of the Korean Peninsula in the context of regional security," Guterres said.

While the leaders of the two Koreas have been building ties, diplomacy between the U.S. and North Korea had gotten nowhere since the historic June summit between President Donald Trump and Kim Jong Un.

Nonetheless, although Kim didn't commit to give up his arsenal of nuclear weapons and ballistic missiles that pose a threat to the U.S. mainland during this week's summit, he did make an offer that has kick-started new diplomatic efforts between the U.S. and North Korea.

Kim promised to dismantle North Korea's main rocket launch site in the presence of international experts, and offered to shutter its Nyongbyon nuclear site. That's where the North has a plutonium reactor and a uranium enrichment facility that can produce fissile material for atomic bombs.

The North Korean leader stressed, however, that the U.S. needs to take unspecified "corresponding steps" — possibly declaring a formal end to the 1950-53 Korean War, which ended without a peace treaty, or easing up on sanctions that are hurting its struggling economy — which could pose difficulties for the Trump administration.

Guterres noted that U.S. Secretary of State Mike Pompeo is ready to start negotiations again with North Korea, and said he is "impressed" by the "courage" and "determination" of Trump and Kim Jong Un.

"I hope that it will produce the result we all want" on denuclearizing the Korean peninsula, he said.

Guterres said the United Nations will be "at the disposal" of the United States and North Korea if they consider that the International Atomic Energy Agency can play a role, or the U.N. can help in any other way.

"We are not creating a parallel track," he stressed. "I think that when you have too many parallel tracks you just complicate things. I think now all the effort must be to support those that are in the middle of negotiations to be successful in their efforts."

Guterres spoke to reporters ahead of next week's annual gathering of world leaders at the U.N. General Assembly and said he would receive North Korea's Foreign Minister Ri Yong-ho, just as he did last year.

Asked whether there was a possibility that Kim Jong Un might show up he shrugged his shoulders and said later: "I have no indication whatsoever of any change in the Korean delegation."