The two men met in Singapore in June and signed a vaguely worded document that pledged to "work toward" the complete denuclearization of the Korean peninsula, though it did not detail specifics on how this would come about.

According to CNN, Pyongyang has upgraded the Yeongjeo-dong missile base in North Korea's mountainous interior and has built up another facility that had not previously been publicly identified.

Experts told the US news network that the site's location made it suitable to host North Korea's newest long-range missiles, including ones that could carry nuclear warheads.

In a statement the Pentagon said: "We watch North Korea very closely but we cannot discuss intelligence."

Trump said on Saturday that he hoped to organize a second summit with Kim in early 2019.

National Security Advisor John Bolton said Tuesday that Trump thinks the North Korean strongman hasn't met the commitments he made at the Singapore summit.

US officials insist on the complete, verified and irreversible denuclearization of the Korean peninsula before tough sanctions are lifted.

But the North has rejected demands for what it calls "unilateral" disarmament, and has instead sought unspecified reciprocal US measures in a gradual process.