President Donald Trump and North Korean leader Kim Jong Un held three high-profile meetings starting in June 2018, after an uptick in tensions at the start of Trump's term.

But nothing concrete emerged in terms of Pyongyang's denuclearization, and talks have been officially at a standstill for months.

"We did have hopes that we could make further progress, that Chairman Kim would go in a different direction, but I'm still optimistic," Pompeo said at a think tank roundtable in Washington.

"It's gone quiet publicly, but there's still lots of work going on... between ourselves and our allies in the region, the Japanese, the South Koreans, and even efforts with the North Koreans to come to understand where there may be opportunity as time goes on."

Trump nevertheless continues to portray his close relationship with Kim as one of his foreign policy achievements.