North Korea’s claim of a successful hydrogen bomb test marks a major step in the isolated country’s long-stated goal of developing a nuclear-tipped missile that puts the U.S. mainland within range, experts say.

The detonation produced 10 times more power than the fifth nuclear test a year ago, South Korean and Japanese officials said.

North Korea’s Nuclear Weapons Institute said Sunday’s test verified the functioning of a hydrogen bomb, including the “fission to fusion power rate and all other physical specifications reflecting the qualitative level of a two-stage thermo-nuclear weapon”, according to the official KCNA news agency.

For the first time, North Korea also specifically mentioned the possibility of a electromagnetic pulse, or EMP, attack. Such a strike would involve detonating a bomb in the atmosphere, instead of firing a long-range missile at a major U.S. city.

Some U.S. policymakers and experts have raised concerns about such an attack, which could cause a massive power surge and deal a devastating blow to U.S. electric grid and critical infrastructure.