SEOUL—North Korea said it conducted a sixth and significantly larger nuclear test Sunday, stepping up pressure on President Donald Trump in what is shaping up to be his biggest foreign policy crisis.

In a televised statement, North Korea described the underground explosion, which triggered a large earthquake, as a “perfect success in the test of a hydrogen bomb for an ICBM.” Pyongyang said “the creditability of the operation of the nuclear warhead is fully guaranteed.”

The test came just hours after leader Kim Jong Un showed off what he described as a hydrogen bomb capable of being mounted on an intercontinental ballistic missile.

The explosion at the nuclear test site at Punggye-ri in North Korea’s mountainous northeast triggered an initial magnitude-6.3 earthquake, followed by a magnitude-4.1 temblor that was possibly caused by a structural collapse, according to the U.S. Geological Survey.

While it was unclear what set off the second quake, satellite imagery has captured evidence of tunneling activity at the nuclear test site in recent months. Any structural collapse might have been related to that.