Washington remained committed to the "complete, verifiable and irreversible" denuclearisation of North Korea, Pompeo added, after the joint statement from the historic US-North Korea summit in Singapore drew criticism for lack of detail on the key issue.

"We believe that Kim Jong Un understands the urgency... that we must do this quickly," he said of the effort to have North Korea abandon its nuclear weapons.

Washington's top diplomat was in Seoul to brief his South Korean and Japanese counterparts on Tuesday's historic talks -- the first between sitting leaders of the two countries -- after which a triumphant President Donald Trump said the world can "sleep well".

"There is no longer a Nuclear Threat from North Korea," he tweeted Wednesday.

Despite the US leader's positive tone, North Korea observers remain concerned that Kim provided no specific guarantees at the summit while notching up a huge publicity win.

And there appeared to be concern in South Korea and Japan too.

Trump said after meeting Kim that the US would halt its "provocative" joint military drills with South Korea as long as negotiations are ongoing with the North, an announcement that caught Seoul -- and apparently the Pentagon -- by surprise.

The US and South Korea conduct massive annual military exercises to maintain readiness for operations on the peninsula, a source of irritation for Pyongyang, which considers them preparations for an invasion.

South Korean Foreign Minister Kang Kyung-wha appeared to sidestep the issue at the joint press conference with Pompeo and her Japanese counterpart Taro Kono, saying the matter would be left to military authorities to discuss, and that the US-South Korea alliance remains "as robust as ever".

Kono said Japan understood the pause in US-South Korea military drills to be contingent on North Korean steps towards denuclearisation. On Wednesday, the Japanese defence minister had described the exercises as "vital" to security in the region.

Pyongyang has in the past linked its drive to acquire nuclear weapons and long-range missiles with the US military presence in the region, describing its weapons programmes as a deterrent.

- 'Save a fortune' -

In an apparent bid to reassure the two allies, Pompeo stressed Thursday that close coordination with Seoul and Tokyo was "crucial" to the success of efforts to denuclearise North Korea.

Pompeo, who said earlier that he hopes to see "major disarmament" of North Korea by 2020, is scheduled to fly to Beijing to meet his Chinese counterpart after his engagements in Seoul.

The Singapore summit is being touted as a victory by the Trump administration, despite criticism that it was more style than substance and had set out no concrete plan for Pyongyang to give up its nuclear weapons.

A bullish Trump said people "can now feel much safer than the day I took office", and defended his position on the military drills with South Korea.

"We save a fortune by not doing war games, as long as we are negotiating in good faith -- which both sides are!" the US leader tweeted on Wednesday.

His comments are in stark contrast to his threats to unleash "fire and fury" last year over Pyongyang's nuclear and long-range missile tests. The North, meanwhile, boasted of its ability to annihilate the United States.

Whirlwind diplomatic efforts, kicked off earlier this year by the Winter Olympics, helped ease tensions and culminated with the Singapore summit, which is also being played up as a major victory by the Kim regime, which has long craved international legitimacy.