Pompeo says that US sanctions on the North will remain in place until the North denuclearizes.
He added: "If diplomacy does not move in the right direction, those measures will increase". The North Korean leader's vehicle was surrounded by running body guards while at a summit with American President Donald Trump in Singapore.
North Korea's Kim Jong Un and U.S. President Donald Trump arrived in the tropical city-state on Sunday for the historic meeting, which will be the first time leaders of the two countries have come face-to-face.
He would not say whether that included the possibility of withdrawing US troops from the Korean Peninsula, but stressed the context of the discussions was "radically different than ever before".
Kim Jong-un grinned for a photo with Singapore's finance minister Vivian Balakrishnan and education minister Ong Ye Kung atop the famous Gardens by the Bay landmark in the Asian city. He is turning 72 on June 14.
Kim was heard telling Trump through a translator: "I think the entire world is watching this moment".
It is not known whether Trump will make any mention of Pyongyang's appalling human rights record or only lavish Kim with praise, something that critics have said plays into the leader's goal of using the summit to boost his own global legitimacy.
The meeting was Trump's only scheduled public event today.
Kim spent the day mostly out of view - until he left his hotel for a late-night tour of Singapore sights, including the Flower Dome, billed as the world's biggest glass greenhouse.
This is the furthest a reigning North Korean President has ever travelled from Pyongyang in the country's history, and it appears Mr Kim used the opportunity to take in some of the sights.
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Whether Trump and Kim will be able to bridge the gap on this key issue remains the key question ahead of the summit.
If there is "tangible progress" on denuclearization, Seoul-based analysts said Washington should compensate Pyongyang for taking steps for abandoning nuclear arsenal by providing security guarantees for the Kim Jong-un regime.
Pompeo has not said if Trump would consider withdrawing almost 28,000 US troops from the Korean Peninsula.
Trump had nicer things to say about Kim than about some of the western allies he met with over the weekend at the G-7 summit in Canada, particularly Canadian Prime Minister Justin Trudeau.
In return, he said, "We will take actions to provide them sufficient certainty that they can be comfortable that denuclearisation is not something that ends badly for them", Pompeo said.
Moon may deliver a message on the DPRK-U.S. summit following the results of the meeting, presidential spokesperson Kim Eui-kyeom said.
A working lunch is on the agenda after this meeting. The official was not authorized to speak publicly about internal deliberations and insisted on anonymity.
"We've got a very interesting meeting ... tomorrow, and I just think it's going to work out very nicely", Trump said.
He brought his own pens and pencils to the Panmunjom meeting and his staff wiped off anything he touched so as not leave even a fingerprint, according to the New Yorker magazine. "In light of how many flimsy agreements the United States has made in previous years, this president will ensure no potential agreement will fail to adequality address the North Korean threat".