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.”

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