US Secretary of State Mike Pompeo said Monday he expects to meet with Kim Jong Un soon to flesh out details of the North Korean leader's promise of nuclear disarmament.

As Washington's top diplomat, Pompeo laid much of the groundwork for last week's historic summit between Kim and President Donald Trump, and is now planning follow-up talks.

Addressing business leaders at the Detroit Economic Club, Pompeo said it was "hard to know" if and when the two heads of state would hold another summit to finalize a deal.

But more discreet diplomacy is continuing.

"There's a lot of work between here and there. My team is already doing it. I'll likely travel back before too terribly long," Pompeo said.

"We still need to flesh out all the things that underlay the commitments that were made that day in Singapore."

Before the Singapore summit, Pompeo had met twice with Kim Jong Un in Pyongyang, once secretly in his previous post as director of the CIA.

Many experts and Trump critics have suggested Kim came away from Singapore having won cheap US concessions in exchange for a vague promise of future disarmament.

But Pompeo insisted that Kim is serious.

"He has made very clear his commitment to fully denuclearize his country," Pompeo said. "That's everything, right? it's not just the weapon systems, it's everything.

"In return for that, the president has committed to making sure that we alter the armistice agreement to provide the security assurances that Chairman Kim needs."

Separately, Pompeo's office said, the secretary spoke to his South Korean counterpart Foreign Minister Kang Kyung-wha "to discuss next steps in the wake of the historic Singapore summit."

Earlier, Kang had said that sanctions against North Korea could be eased as soon as it takes "substantive steps towards denuclearization," seemingly setting the bar lower than Washington.

But Pompeo's office said both allies remain "committed to the goal of complete, verifiable, and irreversible denuclearization."

According to the Russian foreign ministry, Pompeo also spoke about North Korea by telephone with Foreign Minister Sergei Lavrov.

Lavrov and Pompeo discuss North Korea
Moscow (AFP) June 18, 2018 –

Russian Foreign Minister Sergei Lavrov and his US counterpart Mike Pompeo have discussed North Korea by phone less than a week after a historic meeting between US President Donald Trump and North Korean leader Kim Jong Un, Moscow said Monday.

Lavrov and US Secretary of State Pompeo discussed the "task of consolidating efforts towards a solution to problems of the Korean peninsula," the Russian foreign ministry said in a statement on its website.

The exchange comes six days after Trump and Kim made history held a summit in Singapore in an unprecedented encounter that saw the two leaders shake hands.

The Kremlin welcomed the meeting, with President Vladimir Putin saying it was "without doubt just the first step towards a full-blown settlement."

"Thanks to this meeting a possible negative scenario has been cast aside," Putin said.

On Thursday the Russian leader reiterated a invitation for Kim Jong Un to visit Russia as he was hosting North Korea's ceremonial head of state Kim Yong Nam.

Putin said he would be happy to welcome Kim to Russia, suggesting they meet during an economic forum in the far eastern port city of Vladivostok this September.

Lavrov and Pompeo also discussed "the calendar of political contacts between Russia and the US in the near future," according to the foreign ministry statement.

On June 10, Putin said he was ready to meet his US counterpart as soon as Washington was ready, adding Vienna could be a possible venue for such a summit.

The last, brief meeting between Putin and Trump took place in November 2017 in Vietnam during an APEC leaders summit.