Claiming a significant advance in its strike capability, North Korea said it successfully tested a miniaturized hydrogen nuclear bomb on Wednesday, setting off alarm bells in Japan and South Korea.
South Korean intelligence officials and several analysts questioned whether Wednesday's explosion was indeed a full-fledged test of a hydrogen device, reported Reuters.
The test, the fourth time the isolated state has exploded a nuclear device, was ordered by young leader Kim Jong Un and successfully conducted at 10:00 a.m. local time (0130 GMT), said North Korea's official KCNA news agency.
"Let the world look up to the strong, self-reliant nuclear-armed state," Kim wrote in what North Korean state TV displayed as a handwritten note.
But the reported nuclear test drew condemnation abroad, including from China and Russia, North Korea's two main allies.
China, which said it did not have advance knowledge of a test, expressed "resolute opposition" and said it would lodge a protest with Pyongyang.
While a fourth nuclear test had been long expected, the claim that it was a hydrogen device, much more powerful than an atomic bomb, came as a surprise, as did the timing. It ensures that North Korea will be a key topic during the U.S. presidential campaign.
North Korea has long coveted diplomatic recognition from Washington but sees its nuclear deterrent as crucial to ensuring the survival of its third-generation dictatorship.
"With Iran being off the table, the North Koreans have placed themselves at the top of the foreign policy agenda as far as nation-states who present a threat to the U.S.", said Michael Madden, an expert on the country's secretive leadership.
The device had a yield of about 6 kilotonnes, according to the office of a South Korean lawmaker on the parliamentary intelligence committee - roughly the same size as the North's last test, which was equivalent to 6-7 kilotonnes of TNT.
"Given the scale, it is hard to believe this is a real hydrogen bomb," said Yang Uk, a senior research fellow at the Korea Defence and Security Forum.
"They could have tested some middle stage kind (of device) between an A-bomb and H-bomb, but unless they come up with any clear evidence, it is difficult to trust their claim."
"Because it is, in fact, hydrogen, they could claim it is a hydrogen bomb," he said. "But it is not a true fusion bomb capable of the massive multi-megaton yields these bombs produce".
The United States Geological Survey reported a 5.1 magnitude quake that South Korea said was 30 miles from the Punggye-ri site where the North has conducted nuclear tests in the past.
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