The government slammed North Korea on Wednesday after it apparently conducted another nuclear test, its fourth, which involved what Pyongyang described as a powerful hydrogen bomb.

Prime Minister Shinzo Abe denounced the action as “a grave threat to the safety of our country.”

There were stern responses from Seoul and Washington, and China, North Korea’s sole major ally, said it “firmly” opposes the blast.

A statement distributed by the state-run Korean Central News Agency declared that the nation has now “proudly joined the advanced ranks of nuclear weapons states.” It cited the threat posed by U.S. nuclear forces in the region and dubbed the weapon an “H-bomb of justice.”

Japanese officials said Wednesday they were trying to confirm whether the device was indeed a hydrogen bomb, which is proportionally more powerful than a conventional nuclear bomb but which is much harder to develop.

Abe called the test “absolutely intolerable.”

“This is a clear violation of the past resolutions of the United Nations Security Council, and a grave challenge against the international efforts to prevent nuclear proliferation,” he said.

Japan will take “decisive actions” in coordination with the United States, South Korea, China and Russia, he said.

On Jan. 1, Japan took up a nonpermanent seat on the United Nations Security Council. It also is currently holding the rotating presidency of the Group of Seven leading economies.

Foreign Minister Fumio Kishida said Tokyo will ask the U.N. Security Council to convene an urgent meeting and adopt a new resolution on North Korea.

He called Pyongyang’s announcement “a provocative action” that violates a number of U.N. Security Council resolutions and represents a “grave challenge” to peace and safety in the region.

Later in the day, Kishida met with U.S. Ambassador Caroline Kennedy at the Foreign Ministry in a gesture seen as presenting a united front.”We stand with Japan and our partners” in dealing with North Korea, Kennedy said in comments heard by reporters as the meeting began.

Wednesday’s test is North Korea’s fourth. It first detonated a nuclear device in October 2006.

A hydrogen bomb, which uses nuclear fusion in a chain reaction, causes a far more powerful explosion than the nuclear fission generated by uranium or plutonium alone.

Analysts say if the claim is true, it represents a new balance in the region, as North Korea may now be more of a threat to neighbors such as Japan.

It would also mean Pyongyang possesses such advanced technology that even Beijing — its main ally — cannot now rein in its nuclear advances, said Hideshi Takesada, a professor at Takushoku University and an expert on North Korea.”Technologically speaking, it is possible the North has succeeded in developing a hydrogen bomb, as it has claimed,” Takesada said, noting that the assertion is still unverified.

Pyongyang has been trying for years to diversify its nuclear arsenal and the means of delivering warheads, Takesada said. It is thought to possess both plutonium — and uranium-based atomic bombs, and is known to have both mid — and long-range ballistic missiles.Furthermore, he noted recent reports of attempts to develop a submarine-launched rocket.

This means it is logical that the North should try to develop a far more powerful hydrogen bomb, the production of which is more difficult to detect and monitor from overseas than regular atomic bombs, Takesada said.

The implications for Japan would be “enormous,” he said.

A high-ranking Japanese official, speaking on condition of anonymity, echoed Takesada’s concerns.

“It is Japan that will be exposed to the biggest threat if the North develops a (new) nuclear weapon,” the official said.

The official said North Korea is now more unpredictable than ever, as it is more isolated than in the past and both Beijing and Moscow wield less clout in Pyongyang.

“The North seems to be highly unpredictable now. That’s scary,” the official said.

Meanwhile, Wednesday’s development is likely to stall talks over Japanese citizens who were abducted by North Korean agents in the 1970s and 80s and are believed to be living there.

Japan has long tried to persuade Pyongyang to return them with the offer of potential economic assistance. Pyongyang has said the individuals are either dead or never entered the country.

The nuclear test will make it almost impossible for Tokyo to resume talks, the official said.

“Japan won’t ease its actions against the North” because of the abduction issue, the official said. “Now it’s become difficult to have talks with the North.”

Tatsujiro Suzuki, a professor at Nagasaki University’s Research Center for Nuclear Weapons Abolition, is skeptical of Pyongyang’s claim that it has developed and denoted a hydrogen bomb.

The magnitude of Wednesday’s explosion seems to be about the same as those of atomic bomb tests that the North conducted in the past, he pointed out.

A hydrogen bomb usually releases about 1,000 times more energy than an atomic bomb of the same size when it explodes, Suzuki said.

“(Wednesday’s blast) is too small for hydrogen bomb,” he said, adding Pyongyang may have developed a small, non-hydrogen nuclear device.

According to the Metrological Agency’s estimate, the tremor observed at northeastern North Korea on Wednesday had a recorded magnitude of 5.1.

Yonhap News Agency meanwhile said the magnitude of temblors from the North’s past atomic bomb tests were 3.7 in 2006, 4.5 in 2009 and 4.9 in 2013.

“The countries that did developed an atomic bomb produced a hydrogen bomb shortly thereafter, about five or seven years later. So I don’t think it’s impossible (for North Korea to develop one),” Suzuki said.

Still, the North would need large amounts of funds and materials to carry out a project of this scale. Given the country’s circumstances, “it may be too early for North Korea” to develop a hydrogen bomb, he said.

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