Analysts say the North Korea has been continuing to refine its weapons capabilities more than a year after a summit between Kim Jong Un and Trump broke down in Hanoi.
Negotiations have since been deadlocked over sanctions relief and what the North Korea would be willing to give up its nuclear weapons in return, despite a high-profile meeting between Kim and Trump last June in the Demilitarized Zone （DMZ） that divides the Korean peninsula.
Pyongyang set a unilateral deadline for Washington to offer fresh concessions by the end of 2019, and in late December Kim declared his country no longer considered itself bound by moratoriums on nuclear and intercontinental ballistic missile tests.
A region on the Korean peninsula that demarcates North Korea from South Korea.
An explosive device that derives its destructive force from nuclear reactions.
U.S.-led diplomatic efforts aimed at getting the North Korea to give up its nuclear weapons have been stalled for months.