The missile base activity and construction shown by the satellite imagery over Yeongjeo-dong occurred after Mr. Kim agreed during a major summit with President Trump in Singapore in June to abandon North Korea's nuclear weapons program.
Despite that site being known to the USA intelligence community, images show in the past year, North Korea has significantly expanded to a nearby base.
Lewis and Schmerler said they were still not sure whether Yeongjeo-dong and the new facility under construction in nearby Hoejung-ni, both in the mountainous area near North Korea's central border with China, were separate bases or parts of a larger single operation.
As of August 2018, researchers said North Korea is continuing to construct an extremely large underground facility.
The developments come against a backdrop in which Mr. Trump and his advisers say they are hoping for a second summit with Mr. Kim to discuss progress - or the lack thereof - toward denuclearization.
"Whatever Kim says about his desire for denuclearization, North Korea continues to produce and deploy nuclear armed missiles".
In a statement the Pentagon said: "We watch North Korea very closely but we can not discuss intelligence".
North Korean Foreign Minister Ri Yong-ho arrived in China on Thursday on a three-day trip to secure Beijing's support for an easing of sanctions amid the continuing stalemate of denuclearisation talks with the US.
"Meeting Kim again only validates Kim's strategy of using Trump to play for time and sanctions relief, and keep North Korea on the pathway to becoming a de facto nuclear weapon state", Medeiros told The New York Times.
North Korea has expanded a key long-range missile base in the months since a summit between the country's leader Kim Jong-un and US President Donald Trump, according to satellite images published by CNN on Wednesday, December 5.
A series of United Nations resolutions require North Korea to give up its ballistic missile program.
Bolton's comments Tuesday underscored the lack of progress the United States has made in moving North Korea closer to the complete, verifiable and irreversible denuclearization Washington is seeking, and the concessions it has been making to Pyongyang, even as Trump has stressed the warmth of his relationship with Kim.
Kang is on a visit to the US as the head of the South Korean delegation to attend the funeral of former US President George H.W. Bush, Yonhap news agency reported. On Saturday, he told reporters that his next meeting with Kim was likely to happen in January or February.
One such facility is the missile base near Yeongjeo-dong, a site that has long concerned US and South Korean officials and the subject of the analysis of the new images released Wednesday.
At the time, some experts said the United States could soon accept a North Korean request for jointly declaring the end of the 1950-53 Korean War as part of security assurances to the North.
Many of these bases, including the Yeongjeo-dong facility, have been known to outside analysts.