Behind the Trump administration’s sudden urgency in dealing with the North Korean nuclear crisis lies a stark calculus: a growing body of expert studies and classified intelligence reports that conclude the country is capable of producing a nuclear bomb every six or seven weeks.

That acceleration in pace — impossible to verify until experts get beyond the limited access to North Korean facilities that ended years ago — explains why President Trump and his aides fear they are running out of time. For years, American presidents decided that each incremental improvement in the North’s program — another nuclear test, a new variant of a missile — was worrisome, but not worth a confrontation that could spill into open conflict.

Now those step-by-step advances have resulted in North Korean warheads that in a few years could reach Seattle. “They’ve learned a lot,” said Siegfried S. Hecker, a Stanford professor who directed the Los Alamos weapons laboratory in New Mexico, the birthplace of the atomic bomb, from 1986 to 1997, and whom the North Koreans have let into their facilities seven times.