Nuclear-armed missiles have only been tested a handful of times, said Vipin Narang, a professor of political science at MIT and expert on deterrence and nuclear policy.

“The worst-case scenario would be a Juche bird test,” Narang said. “That would be not only provocative, but a lot can go wrong if the missile goes awry.”

The phrase “Juche bird,” used by nonproliferation experts monitoring Pyongyang’s activities, is a play on the 1962 missile test codenamed Frigate Bird. The test involved a nuclear-tipped missile fired from a US submarine toward Christmas Island and detonated over the Pacific Ocean.