North Korea said it conducted a hydrogen bomb test in a surprise announcement that caused regional tension and drew swift condemnation, while triggering scepticism that such a powerful weapon was actually detonated.
If confirmed, the test would be the fourth time Pyongyang has exploded a nuclear device.
South Korean intelligence officials and several analysts, however, reportedly questioned whether Wednesday's explosion was indeed a full-fledged test of a hydrogen device.
No radiation was detected at Japanese monitoring posts.
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"A real hydrogen bomb would be 50 to 100 times the power of the bombs on Hiroshima and Nagasaki," nuclear scientist Imad Khadduri told Al Jazeera. "The yield of this bomb is nowhere near that."
The test was met with a burst of jubilation and pride in the capital Pyongyang. A North Korean television anchor, reading a typically propaganda-heavy statement, said a test of a "miniaturised" hydrogen bomb had been a "perfect success" that elevated the country's "nuclear might to the next level".
Before the announcement was made on state television, South Korea's meteorological agency said a 5.1 magnitude earthquake was detected near a known test site in the secretive and isolated state.
South Korea's presidential office held an emergency meeting and later said the government would take all possible measures to respond to its neighbour and long-time foe's actions.
"Our government strongly condemns North Korea ignoring repeated warnings from us and the international community and pushing ahead with the fourth nuclear test, which clearly violated the UN resolutions," Cho Tae-yong, a senior security official at the South Korean presidential office said.
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China's foreign ministry said it had no advance knowledge of any test and it firmly opposed Pyongyang's action.
Hua Chunying, a ministry spokeswoman, said at a daily news briefing that Beijing - one of North Korea's only allies - would work with the international community on the denuclearisation of the Korean peninsula.
The US government said it could not confirm a nuclear test but it would respond appropriately to what it called North Korean "provocations" and would continue to protect its allies in the region.
"We have consistently made clear that we will not accept [North Korea] as a nuclear state," a State Department statement said.
Japanese Prime Minister Shinzo Abe called the announcement a threat to his nation's safety.
Abe told reporters: "We absolutely cannot allow this, and condemn it strongly."
International Atomic Energy Agency chief Yukiya Amano's said in a statement the North's nuclear test "if confirmed is in clear violation of UN Security Council resolutions and is deeply regrettable".
The United States Geological Survey (USGS) also detected the quake that South Korea said was 49km (30 miles) from the Punggye-ri site where the North has conducted nuclear tests in the past.
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While the USGS put the depth of the earthquake at 10km, the South Korean agency said it was near the surface. The earthquake was detected just after 10am Seoul time (1am GMT).
Al Jazeera's Adrian Brown, reporting from Beijing, said that North Korea was moving one step nearer to creating a nuclear warhead.
"There will be a good deal of tension once more on the Korean Peninsula today. The big question of course is why North Korea has done this now," he said.
Brown added the test could partly be seen as a defensive response to US-South Korean military exercises, as well as a reaction to UN sanctions the country is under as a result of for its nuclear and missile programmes.
"When North Korea last tested a nuclear device in 2013, that was as a direct consequence of the sanctions that had been imposed by the United Nations," Brown said.
North Korea has so far conducted three nuclear tests - in 2006, 2009 and 2013 - all at Punggye-ri.
The 2013 test registered at 5.1 on the USGS scale.
It is not yet known if Pyongyang has successfully made a nuclear device small enough to be used as a warhead on a ballistic missile, but the likelihood of the isolated country successfully doing so increases with each test.
Nuclear experts said the yield of the blast did not reflect the size of a real hydrogen bomb.
An estimated explosive yield of 6.0 kilotons and a quake with a magnitude of 5.1 were detected, South Korean lawmaker Lee Cheol Woo said its National Intelligence Service told him.
That's smaller than the estimated explosive yield of 7.9 kilotons reported after the 2013 nuclear test, he said, adding even a failed H-bomb detonation typically yields tens of kilotons.
Source: Al Jazeera and agencies