SEOUL/WASHINGTON (Reuters) - U.S. Secretary of State Mike Pompeo met North Korean officials in Pyongyang on Friday, hoping to “fill in” details on how to dismantle the North’s nuclear program and recover the remains of U.S. troops missing from the Korean War.

Pompeo met Kim Yong Chol, who played a key role with Pompeo in arranging last month’s summit between U.S. President Donald Trump and North Korean leader Kim Jong Un in Singapore, according to a pool report by reporters traveling with him.

The talks lasted nearly three hours and it was uncertain whether Pompeo would meet Kim Jong Un. He will spend the night in Pyongyang, his first overnight stay in North Korea.

“We just wrapped our first meeting of the day,” Pompeo told reporters. “I’m proud of my team’s work.”

At the Singapore summit, Kim Jong Un made a broad commitment to “work toward denuclearization”, but fell short of details on how or when he would dismantle North Korea’s nuclear program, which it has pursued in defiance of U.N. Security Council resolutions.

“The President told me he believes that Chairman Kim sees a different, brighter future for the people of North Korea. We both hope that’s true,” Pompeo said on Twitter after a phone call with Trump as he headed for North Korea.

“Next stop: Pyongyang. I look forward to continuing my meetings with North Korean leaders. There’s much hard work ahead but peace is worth the effort.”

Pompeo said he was seeking to “fill in” some details on North Korea’s commitments and maintain the momentum towards implementing the agreement from the summit, according to the pool report.

Pompeo would try to agree on at least an initial list of nuclear sites and an inventory that could be checked against the available intelligence, U.S. intelligence officials told Reuters.

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Also high on the agenda is the issue of the remains of U.S. soldiers missing from the 1950-53 Korean War. Trump said after the Singapore summit that Kim had agreed to send the remains back to the United States.

Both issues are considered essential tests of whether Kim is serious about negotiations. North Korean officials have yet to demonstrate that in working-level talks, the intelligence officials said.

“If they’re serious, then we can get down to the business of defining the terms of final denuclearization,” said one official.

But the U.S. ability to verify the accuracy of any North Korean list is limited due to the lack of a “high confidence” accounting of the North’s nuclear arsenal, such as the number of warheads and uranium enrichment facilities, especially if they are not operational, they said.

While, in the past, the Pentagon has said North Korean officials have indicated they had the remains of as many as 200 U.S. troops, a U.S. military official familiar with the procedures for handling remains said it was not clear what North Korea might hand over.

“BUILDING TRUST”

Pompeo will stay in the Paekhwawon, or 100 Flowers Garden, a prestigious guesthouse where former South Korean President Roh Moo-hyun stayed during his summit with Kim Jong Un’s father, Kim Jong Il, in 2007.

“This is your third visit to our country so I guess you are used to it now,” Kim Yong Chol told Pompeo at the guesthouse, according to the report. “The more you come, more trust we can build between one another.”

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Some officials in the State and Defense Departments and in U.S. intelligence agencies are worried that Trump has put himself at a disadvantage by overstating the results of the Singapore summit.

Ahead of the summit, Pompeo said Trump would reject anything short of “complete, verifiable and irreversible denuclearization”.

But following talks on Sunday between U.S. envoy Sung Kimand North Korean counterparts, this “CVID” language appears to have disappeared from the State Department lexicon.

It says pressure will remain until North Korea denuclearizes, but in statements this week, it redefined the U.S. goal as “the final, fully verified denuclearization” of the country.

Some U.S. officials and experts have said the change in language amounted to a softening in approach. The State Department said its policy remains unchanged.

Pompeo’s talks, which resume on Saturday, will be closely watched in the region. He is due to meet officials from allies South Korea and Japan in Tokyo also on Sunday.

A spokesman for South Korea’s presidential office would only say South Korea and the United States were working to formulate “constructive measures” on North Korea’s denuclearization.