North Korea conducted its fifth and most powerful nuclear test to date around 9:30 a.m. Friday, prompting Tokyo to immediately file a protest amid concerns over accelerating developments in the North’s nuclear and ballistic missile technologies.

Tokyo lodged the protest through its embassy in Beijing, saying the test posed a “grave threat” to Japan, the region and international society, Chief Cabinet Secretary Yoshihide Suga told a news conference.

The United States and Japan are jointly urging the United Nations Security Council to convene an emergency meeting to condemn the North, Suga said.

“Scientists and technicians of the DPRK carried out a nuclear explosion test for the judgment of the power of a nuclear warhead newly studied and manufactured by them at the northern nuclear test ground,” the state-run Korean Central News Agency (KCNA) said Friday.

DPRK is the acronym for the Democratic People’s Republic of Korea, the formal English name of North Korea.

“The nuclear test finally examined and confirmed the structure and specific features of movement of nuclear warhead that has been standardized to be able to be mounted on strategic ballistic rockets,” the KCNA claimed.

Pyongyang is known for its regular use of overblown rhetoric and few experts take any of its official statements at face value.

But the country is believed to already possess dozens of intermediate ballistic missiles that can directly hit Japan, and to have already conducted five nuclear explosion tests so far.

If Pyongyang succeeds in producing a nuclear warhead small enough to be mounted on those ballistic missiles, the threat to Japan, and possibly to the U.S. as well, will dramatically increase.

On Friday, Defense Minister Tomomi Inada did not rule out such a scenario for Tokyo.

“You cannot deny North Korea might have managed to make a nuclear warhead small enough to fit a ballistic missile,” Inada said.

Later in the day, Prime Minister Shinzo Abe held a teleconference with U.S. President Barack Obama, who was aboard Air Force One on his way home from Laos.

During the 10-minute conversation, Obama told Abe he wants to make clear to Japan that the U.S. commitment to defend Japan is “unshakable,” according to a senior Japanese official who briefed reporters in Tokyo.

Japan is a military ally to the U.S., which is obliged under a bilateral security treaty to defend Japan if it is attacked by a third country.

Abe, meanwhile, told Obama that the nuclear test violates U.N. Security Council resolutions and is totally unacceptable to Japan.

The international community should take “a resolute response” and have the North pay “a price for its provocative actions,” Abe told Obama, according to the Japanese official.

Obama then said he “completely agreed” with Abe, the official said.

On Friday morning, the Japan Meteorological Agency detected a magnitude-5.3 earthquake originating around Punggye-ri — a known nuclear test site — which is in the northeastern part of North Korea.

According to the agency, the focus of the quake was just under the surface of the Earth, which indicated the earthquake might have been artificially created.Sometime later, Pyongyang declared it had carried out a fifth nuclear explosion test following a previous one on Jan. 6.

The magnitude-5.3 temblor that was generated was stronger than any produced by the other four nuclear tests North Korea has carried out.

Officials in Tokyo expressed concern that the North appears to have made considerable progress in nuclear and ballistic missile development in recent months.

On Monday, during the Group of 20 summit attended by world leaders in the city of Hangzhou, in eastern China, North Korea test-fired three ballistic missiles into the Sea of Japan, and they landed within Japan’s exclusive economic zone off Hokkaido after flying about 1,000 km.

“The three ballistic missiles were precisely controlled. Then this nuclear test followed,” a senior Japanese official said Friday in Tokyo.

“We have to cope with the situation” assuming Pyongyang might have made considerable progress in developing a nuclear warhead small enough to fit on a ballistic missile, the official said, speaking on condition of anonymity.

North Korea observed its national foundation day on Friday, and experts said Pyongyang might have chosen that day for the test in a bid to boost the morale of North Korean citizens.

“That may be one factor. But we cannot say anything for certain as yet,” Inada said.

During a news conference, Suga said Japan will consider slapping new economic sanctions on Pyongyang and will urge the U.N. Security Council to adopt a new resolution to punish the North.

But Masao Okonogi, professor of Korean studies at Kyushu University, is skeptical of the effectiveness of any new sanctions given all past measures have failed to stop North Korea’s nuclear and missile development programs.

“We are running out of measures to stop North Korea,” Okonogi told The Japan Times on Friday.

“The U.N. will probably adopt a new resolution, but won’t be able to take any military action. All possible economic sanction measures have been already taken.”

China is North Korea’s main trading partner and many diplomats argue that Beijing should ratchet up pressure on Pyongyang by threatening to cut off trade, in particular oil supplies.

But if China pulls the plug on oil supplies to the North, the leadership of the hermit country could “go out of control” in despair, Okonogi warned.

“Economic sanctions are better than nothing, but they won’t solve all the problems,” Okonogi said.

“It’s true China thinks denuclearization (of the North) is desirable. But Beijing doesn’t want the North to collapse at the same time. So you shouldn’t expect too much” from any action taken by China, he said.

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