North Korea’s threats against the U.S. now include a tactic long discussed by some experts: an electromagnetic pulse, or EMP, triggered by a nuclear weapon that would aim to shut down the U.S. electricity grid.

North Korea’s state news agency made a rare reference to the tactic in a Sunday morning release in which the country said it was able to load a hydrogen bomb onto a long-range missile. The bomb, North Korea said, “is a multifunctional thermonuclear nuke with great destructive power which can be detonated even at high...