SEOUL, South Korea (AP) — North Korea says it has conducted a hydrogen bomb test. The surprise announcement complicates already difficult efforts to curb the country’s push for a working nuclear arsenal.

The North said Wednesday in a broadcast that the test was successful. A hydrogen bomb is generally more powerful than a nuclear bomb.

North Korea has long pushed for an arsenal of warheads that can be mounted on a missile capable of reaching mainland America’s shores. North Korea is thought to have a handful of rudimentary nuclear bombs. But it is not yet thought to have succeeded.

South Korean officials earlier detected an “artificial earthquake” near North Korea’s main nuclear test site.

An H-bomb test will almost certainly lead to a push for new sanctions at the United Nations.