The warheads — at least 20 in number, and perhaps as many as 60 — remain for now in their bunkers, somewhere in the rugged hills north of Pyongyang. Until today, there has been no public pledge from North Korea to dismantle them, or to allow inspectors to see them, or even to disclose where they are kept.

Work continues daily in the country’s radiochemistry lab near Yongbyon, where plutonium for new bombs is extracted from spent fuel rods. Just across a small river from the lab, testing continues on a 20-megawatt reactor capable of producing nuclear fuel for scores of additional bombs.

The facilities are among hundreds that exist across a North Korean weapons complex that has shown itself capable not only of making sophisticated nuclear and chemical weapons, but also of expertly hiding them from public view. It is why weapons experts around the world expressed astonishment Wednesday at President Trump’s claim that the danger posed by Pyongyang’s decades-long weapons buildup had been effectively eliminated — that there was, as Trump wrote in a Twitter posting, “no longer a nuclear threat from North Korea.”