North Korea says it has successfully conducted test of hydrogen bomb meant for an intercontinental ballistic missile.
A U.S. official who studies North Korea's military and politics said that seismic data on the tremors was being analysed, although the location suggested another nuclear test.
The blast, which came almost a year after the North's fifth nuclear test, appeared to be the most powerful yet, according to South Korea's weather agency.
Seismological data from the United States Geological Survey (USGS) showed that an explosion caused a 6.3-magnitude tremor in the country's northeast, not far from the country's Punggye-ri nuclear test site.
The previous nuclear blast in North Korea is estimated by experts to have been around 10 kilotons.
It was the sixth underground bomb tested since 2006, North Korea's state-run TV broadcaster said.
North Korea has developed a hydrogen bomb which can be loaded into the country's new intercontinental ballistic missile, the official Korean Central News Agency claimed today.
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Japan said it had concluded there was a nuclear test.
Italian Foreign Minister Angelino Alfano said in a statement that North Korea must immediately abandon its nuclear and missile development program.
There was no independent confirmation of the claims.
Just before the test, according to state media, Kim and the other senior leaders at the party presidium meeting discussed "detailed ways and measures for containing the USA and other hostile forces' vicious moves for sanctions".
The U.S. on Thursday had sent the bombers and F-35 stealth fighters to the skies of South Korea in a show of force - and North Korea strongly protested.
The developments are raising already high tensions on the Korean Peninsula and in Washington that the North is getting closer to its goal of an arsenal of intercontinental ballistic missiles (ICBMs) that can reach the U.S. mainland.
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The bomb was created to be mounted on its newly developed intercontinental ballistic missile (ICBM) the North said in the announcement, which came hours after the US Geological Survey (USGS) reported a 6.3 magnitude quake.
It is seeking to develop a nuclear weapon small enough to be mounted on an ICBM.
Any potential United States move for military action, however, would prompt resistance from the Moon Jae-in government, which has been calling for dialogue as a first and foremost solution to the crisis.
The shape shows a marked difference from pictures of the ball-shaped device North Korea released in March past year, and appears to indicate the appearance of a two-stage thermonuclear weapon, or a hydrogen bomb, said Lee Choon-geun, senior research fellow at state-run Science and Technology Policy Institute.
The US has about 28,000 troops stationed in South Korea and is obliged by treaty to defend it in the event of war. He called on North Korea to "immediately cease all existing nuclear and ballistic missile activities in a complete, verifiable, and irreversible manner, and re-engage in dialogue with the global community". But if the North Korean program is advancing at the pace it claims, it may be too late to debate over negotiations or their meaning - the North may be closer than anyone expected to having a viable nuclear deterrent. A key question is how far North Korea has gotten in efforts to consistently shrink down nuclear warheads so they can fit on long-range missiles.
"North Korea has a choice".
"The H-bomb, the explosive power of which is adjustable from tens kiloton to hundreds kiloton, is a multi-functional thermonuclear nuke with great destructive power which can be detonated even at high altitudes for super-powerful EMP [electromagnetic pulse] attack according to strategic goals", KCNA reported in English.
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