The US has said it will not be easing sanctions until on North Korea until it has denuclearised – despite claims made by Pyongyang state media.
Secretary of state Mike Pompeo insisted that Pyongyang was committed to giving up its nuclear arsenal but warned it would "be a process, not an easy one".
He pushed back on reports that the leaders of the US and North Korea had agreed on a "step-by-step" process, which some interpreted as meaning that America would grant concessions along the way.
Mr Pompeo made the comments at a news conference with the Japanese foreign minister Taro Kono and South Korean foreign minister Kang Kyung-wha in Seoul.
In one awkward moment, diplomats ignored a journalist's shouted question about President Donald Trump's decision to halt joint US-South Korean military drills.
The military drills have been earmarked as a point of no concession for South Korea, with Japan's defence minister describing them as "vital" for the region's security.
Mr Pompeo also defended Donald Trump's tweet in which he said North Korea was no longer America's "biggest and most dangerous problem" and that people should "sleep well tonight!".
He insisted that the president's comment had been made with "eyes wide open".
Mr Pompeo added that Mr Trump was referring to the fact that, for the first time in history, a US president had sat down with a North Korean leader and that the pair had a "blunt conversation" about the changes needed for the isolated nation to rejoin the world community.
3:11The US wants to see a "major disarmament" of North Korea within two-and-a-half years – a time frame taking it up to the end of Mr Trump's first term in office in 2020.
Mr Pompeo admitted there was still "a lot of work left to do" following Tuesday's historic meeting between the president and Kim Jong Un in Singapore.
South Korea's top diplomat told Mr Pompeo this is a chance to "seize the momentum toward peace [and] denuclearisation".
More from North Korea
1:54South Korea's president Moon Jae-in told Mr Pompeo the summit has helped the region move "from the era of hostility towards the era of dialogue, of peace and prosperity".
The rival Koreas are holding rare high-level military talks on Thursday to discuss reducing tensions across their heavily fortified border.
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