South Korean officials say they have detected an artificial 5.6 magnitude quake in North Korea and are analyzing whether Pyongyang has conducted its sixth nuclear test.

South Korea’s weather agency said Sunday the unusual seismic activity occurred in Kilju, northern Hamgyong Province. The U.S. Geological Survey called the quake an explosion with a magnitude 6.3.

North Korea conducted its fifth test last year in September. In confirmed, the latest test would mark yet another big step forward in North Korean attempts to obtain a nuclear-armed missile capable of reaching deep into the U.S. mainland.

It came hours after North Korea claimed that its leader has inspected the loading of a hydrogen bomb into a new intercontinental ballistic missile.

The U.S. State Department had no immediate reaction.

North Korea conducted two nuclear tests last year and has since maintained a torrid pace in weapons tests, including flight-testing developmental intercontinental ballistic missiles and flying a powerful midrange missile over Japan.

(AP)