US intelligence officials believe North Korea has been "deceiving" the USA, saying the Hermit Kingdom was bolstering production for nuclear weapons at "multiple secret sites" in the past few months, a report stated.
Secretary of State Mike Pompeo will travel to North Korea on Thursday to encourage Pyongyang to honor commitments the US says it gave during the Trump-Kim summit.
North Korea will have to back up its words with verifiable actions, Bolton said.
"Our goal remains the final, fully-verified denuclearization of the DPRK, as agreed to by Chairman Kim in Singapore", the official said in an emailed statement, which referred to the initials of North Korea's official name: the Democratic People's Republic of Korea.
U.S. Secretary of State Mike Pompeo will travel to the North Korean capital of Pyongyang on Thursday.
Wayne Lee, Steve Herman reported this story for VOA News.
Diplomatic gains have been marred by USA intelligence and satellite images suggesting that the North is moving ahead with its nuclear weapons programs despite Kim's summit promise to work toward the "complete denuclearization" of the divided peninsula.
Experts say Pompeo will have to coax significant reciprocal steps from North Korea that would firmly lock the country into a process of disarmament to justify the suspension of the U.S.
The images, taken by San Francisco-based Planet Labs Inc. and analyzed by researchers at the Middlebury Institute for International Studies (MIIS) in Monterrey, show the construction of the Chemical Material Institute in the North Korean city of Hamhung.
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Doubts over North Koreas intentions have deepened amid reports that it is continuing to produce fissile material for weapons. But the most sensitive reactor is that five-megawatt reactor, Yongbyon.
A USA official told The Associated Press that the Post's report was accurate and that the assessment reflected the consistent view across US government agencies for the past several weeks.
Speaking on national television on Sunday, national security adviser John Bolton spoke about the idea of denuclearization for the North.
"This is not like North Korea cheating or deceiving the US because they've made no commitments". The Post is withholding details at the request of intelligence officials. He also said Trump and Kim should decide whether to move the nuclear weapons out of North Korea to dismantle them or do it inside the country. "I think it would be inappropriate and, frankly, counterproductive to achieving the end state that we're hoping to achieve".
Pompeo is expected to discuss the denuclearization timetable.
Mr. Trump insisted the talks are moving "very quickly" and that he believed North Korea is serious about total denuclearization.
To date, Kim has halted nuclear and missile tests and has destroyed tunnels at the North's nuclear test site, but the authoritarian nation has yet to take concrete steps toward abandoning its weapons programs.
"First, they would want to say that they have lots of intelligence on North Korea and that its relations with the USA would go back to the past if it doesn't take practical disarmament steps", Nam said.
He added: "They know the schedule of satellites going over North Korea". They also say the USA has an unrealistic approach to North Korea's denuclearization.