By January 2016, the country claimed to have exploded a hydrogen bomb in a fourth test, but outside monitors expressed skepticism. Seismic readings suggested an explosive force of four to six kilotons.

Seismic readings of North Korea’s fifth test, in September 2016, however, registered a force of approximately 10 kilotons, according to South Korea’s Defense Ministry.

Technology: missiles could reach continental U.S. by 2026

In 1999, George J. Tenet, then director of the Central Intelligence Agency, said he could hardly overstate his concern about North Korea’s program, warning that the Taepodong-1 missile, with a reach of up to 1,243 miles, could deliver bomb payloads to Alaska and Hawaii.

In the nearly two decades since, the country’s investment in becoming a nuclear weapons power has succeeded despite diplomacy and international sanctions. In 2016, Mr. Kim launched dozens of missiles for tests and as shows of military might. Some missiles could be launched from mobile pads and submarines, making them easier to hide. They could potentially carry nuclear warheads, according to Siegfried S. Hecker, emeritus director of the Los Alamos National Laboratory in New Mexico, birthplace of the atomic bomb.

He and other analysts have said they assume North Korea has designed and demonstrated nuclear warheads that can be mounted on short-range and perhaps medium-range missiles. Writing in September 2016, Dr. Hecker said, “Pyongyang will likely develop the capability to reach the continental United States with a nuclear tipped missile in a decade or so.”