The State Department's point man on North Korea, Stephen Biegun, met Tuesday in Washington with South Korea's special representative for peace and security on the peninsula, Lee Do-hoon, to "further strengthen our close coordination," Pompeo told reporters.

Washington and Seoul have launched a working group to "make sure that we don't talk past each other," the secretary of state noted.

The talks in the US capital came as Seoul and Pyongyang appear to be moving ahead with their rapprochement more quickly than the Americans and North Koreans are making headway on disarming Kim Jong Un's regime.

"We have made clear to the Republic of Korea that we want to make sure that peace on the peninsula and the denuclearization of North Korea aren't lagging behind the increase in the amount of inner relationship between the two Koreas," Pompeo said.

"We view them as tandem, as moving forward together. We view them as important parallel processes, and that working group is designed to make sure they continue to remain that way."

Since the historic summit between Kim and US President Donald Trump in Singapore in June, talks on denuclearization have stalled, with meetings either deemed unproductive, pushed back or cancelled altogether.

A second leaders' summit is expected to take place in early 2019, according to Washington.

But so far, North Korea has taken few concrete steps to abandon its nuclear weapons, and the two sides have sparred over the meaning of a vaguely worded document signed by Kim and Trump on denuclearization of the peninsula.

At the same time, North and South Korea have made multiple symbolic gestures and concrete decisions on reconciliation.

After three summits with South Korean leader Moon Jae-in this year, Kim is expected to make a landmark visit to Seoul, though it appears that the meeting may not take place this year as initially planned by both sides.