They confirm what has become increasingly obvious in the past few months - that Mr Trump's summit with Mr Kim last June has done little to stop the North from consolidating its long-range nuclear capability.
Satellite images viewed by CNN showed that the Yeongjeo-dong missile base and a nearby, previously unreported site were not only active but were being upgraded. Trump lashed out at the New York Times, calling the newspaper's report about North Korea's missile bases "fake news".
The South Korean military said Thursday that Seoul and Washington have long monitored a North Korean missile base that a U.S. media outlet described as an unreported facility capable of launching a long-range ballistic missile.
In a statement the Pentagon said: "We watch North Korea very closely but we can not discuss intelligence".
"Whatever Kim says about his desire for denuclearization, North Korea continues to produce and deploy nuclear armed missiles", he told CNN.
No North Korean leader has travelled to South Korea since the end of the Korean War, which killed millions.
North Korean leader Kim Jong Un has cleared three days later this month to make a historic first visit to Seoul to discuss ridding the Korea peninsula of nuclear weapons, according to a report Wednesday.
"That's why I think the president thinks that another summit is likely to be productive", Bolton said during the Wall Street Journal's CEO conference.
"China may convey [US] messages to North Korea and encourage it to complete denuclearisation ... to show to the United States that it is still honest in denuclearising North Korea".
National Security Advisor John Bolton said on Tuesday that Trump thinks the North Korean strongman hasn't met the commitments he made at the Singapore summit. Analysts said that by relaxing its demands ahead of a second summit, the USA may have blinked first.
Kim announced the dismantlement of a nuclear facility at another site in Nyongbyon at a summit with South Korean leader Moon Jae-in in September. A headquarters area is positioned at the mouth of the valley, the images show. "In front of the pair of drive-throughs, there is a concrete pad that was also covered in dirt", according to the analysis by Lewis and Schmerler.
Researchers were able to locate five entrances to underground tunnels they said may be used to store missiles. "One such facility is the missile base near Yeongjeo-dong". Additional tunnels were also found, as seen in satellite pictures, including a pair of large drive-through shelters capable of holding large ballistic missiles.