North Korea is thought to have carried out its most powerful nuclear weapon test to date hours after saying it had developed an advanced hydrogen bomb of "great destructive power".
An earthquake registering 6.3 was detected in the secretive state on Sunday 75km (45 miles) north-northwest of Kimchaek, where previous tests have been carried out.
The tremors caused were at least ten times as powerful as the last time Pyongyang exploded an atomic bomb a year ago, according to Japan's metereological agency.
It is the sixth time North Korea has exploded an atomic weapon, and represents a direct challenge to US President Donald Trump, who hours earlier spoke to Japanese Prime Minister Shinzo Abe about the "escalating" nuclear crisis in the region.
"It is absolutely unacceptable if North Korea did force another nuclear test, and we must protest strongly," Mr Abe said.
Pyongyang claimed leader Kim Jong-un had inspected a hydrogen bomb meant for a new intercontinental ballistic missile hours before the tremors were felt.
Analysts fear the development could signify a significant step forward in the North's quest for a viable nuclear missile capable of striking anywhere in the US.
South Korea's President is to chair a National Security Council meeting. Its military said it has strengthened its monitoring and readiness while considering a variety of possible responses in collaboration with the US.
North Korea conducted two nuclear tests last year, the last nearly a year ago, on the 9 September anniversary of the nation's founding.
It has since tested two intercontinental ballistic missiles in July and last month, fired a potentially nuclear-capable missile over northern Japan.
Earlier Sunday, photos released by the North Korean government showed Mr Kim talking with his lieutenants as he observed a silver, peanut-shaped device that was apparently the purported thermonuclear weapon destined for an ICBM.
What appeared to be the nose cone of a missile could also be seen near the alleged bomb in one picture, which could not be independently verified and was taken without outside journalists present.
Another photo showed a diagram on the wall behind Kim of a bomb mounted inside a cone.
State media said Kim visited the Nuclear Weapons Institute and inspected a "homemade" H-bomb with "super explosive power" that "is adjustable from tens (of) kiloton to hundreds (of) kiloton."
North Korea's nuclear and missile programme has made huge strides since Mr Kim rose to power following his father's death in late 2011.
The North followed its two tests of Hwasong-14 ICBMs by threatening in August to launch a salvo of its Hwasong-12 intermediate range missiles toward the US Pacific island territory of Guam.
It flew a Hwasong-12 over northern Japan last week, the first such overflight by a missile capable of carrying nukes, in a launch Mr Kim described as a "meaningful prelude" to containing Guam, the home of major US military facilities, and more ballistic missile tests targeting the Pacific.
Agencies contributed to this report
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