North Korea boasts it has conducted a sixth underground nuclear test

Reporting from Seoul â€”  North Korea on Sunday confirmed that it has conducted its sixth underground nuclear test — the detonation of a hydrogen bomb of “unprecedented powerâ€� designed to be transported by an intercontinental ballistic missile.

The latest brash move threatens to heighten an already tense situation on the Korean Peninsula and raises new questions about how the Trump administration will respond to Pyongyang’s latest provocation.

In just over a week, North Korea has test launched at least four ballistic missiles — including one that flew over Japan, causing serious alarm on the island — and boasted about creating a warhead that could, in theory, be used against the United States.

North Korean officials released a statement a few hours after the launch confirming that they had successfully completed testing of major portions of their ideal nuclear system — a missile that, in theory, could reach the United States.

“Today’s event was meaningful, for we’ve reached our goal of completion of national nuclear power,� said an announcement read on North Korean state television.

The statement said the test marked a “significant occasion in attaining the final goal of completing the state nuclear force.�

In a Sunday morning tweet, President Trump called North Korea a “rogue nation� and a threat to the United States.

“North Korea has conducted a major Nuclear Test,� said the tweet. “Their words and actions continue to be very hostile and dangerous to the United States.�

Tremors from the blasts could be felt in Yanji, a Chinese city of 650,000 on the North Korean border, according to China’s official New China News Agency. The agency tweeted a short video of a crystal chandelier, its hanging crystals swinging.

China’s online social networks lit up with news of the tests; a post by China’s earthquake-monitoring authorities announcing the seismic activity garnered nearly 23,000 “likes� and 17,000 comments.

China’s ministry of foreign affairs expressed “resolute opposition� and “intense condemnation� to the latest test.

“Realizing a nuclear-free Korean peninsula and safeguarding systems of nuclear nonproliferation and peace and stability in [China’s] northeast is the firm position of the Chinese side, and the common will of the international community,� the ministry said in a statement.

Others in China expressed concern about possible radiation from the powerful blast.

“What I really care about is the safety of the northeastern provinces,� posted Hu Xijin, editor in chief of the Global Times, a state-run tabloid. “I believe that the country’s emergency response authorities have already moved on this. If there was indeed a nuclear test, I’m most concerned by whether there was a radiation leak, and I believe the government feels the same.�

U.S. and South Korean officials say the detonation caused an unnatural seismic tremor detected by sensors, a tell-tale sign of a nuclear test. Initially, U.S. and Korean officials believed the blast to have occurred in a village in northeastern North Korea known as Punggye-ri — a site closely watched by international nuclear experts.

North Korean state television later confirmed the test occurred at Punggye-ri. The country’s five previous tests, including two last year, also occurred there.

The magnitude of the nuclear test, North Korea’s first since last September, was estimated at 5.6, according to South Korean officials. Other reports put the magnitude at 6.3. The seismic wave occurred about 12:30 p.m.

The latest experiment — a clear violation of international resolutions, though not unexpected by United States officials — raises new concerns that North Korea continues to advance as a nuclear state, despite years of effort by the international community to curb its atomic program.

The detonation was felt just hours after North Korea boasted that a hydrogen bomb had been mounted on a new intercontinental ballistic missile and that leader Kim Jong Un had inspected the device.

North Korea, one of the world’s most isolated and unpredictable states, appears to be violating global norms with increased impunity.

In August, President Trump was blunt in his warning about the nuclear program.

“North Korea best not make any more threats to the United States,� Trump told reporters. “They will be met with fire and fury like the world has never seen.�

Later, he said that North Koreans “better get their act together or they’re going to be in trouble like few nations ever have been in trouble in this world.�

The rogue state is still technically at war with South Korea, a United States ally that has roughly 28,000 American forces stationed on bases, largely within a few hundred miles of the shared Korean border.

Provocations in recent years, under dynastic young ruler Kim, have included numerous ballistic missile tests; the lengthy prison sentence given to an American tourist, who later died after being released; and a land mine incident along the border in 2015 that severely injured two South Korean soldiers.

The test is the latest provocation by the North, which in April paraded a massive battery of military hardware before the world in a recent celebration — including, perhaps, long-range devices capable of striking targets outside Asia.

In July the country test launched what the international community now believes were intercontinental ballistic missiles — devices in theory capable of reaching the United States.

“I’m not shocked by what North Koreans have done in the last few days at all,� said Harry Kazianis, director of defense studies at the Center for the National Interest. “The North Koreans over the last few months have really ramped up their testing schedule, with their missiles. So a nuclear test was going to happen.�

Kazianis added that “we shouldn’t be shocked anymore. They’ve been working toward nuclear weapons. This has been their goal. It’s something they’ve stated for years.�

North Korea, which security experts say could have more than a dozen nuclear devices, first conducted an underground test in 2006. The tests’ power has increased over time, and last year state media reported advances in the miniaturization and manufacturing of nuclear warheads in addition to its strongest experiment to date last September.

“The standardization of the nuclear warhead will enable the DPRK to produce at will and as many as it wants a variety of smaller, lighter and diversified nuclear warheads of higher strike power,� the government said last September, using the initials of North Korea’s formal name, the Democratic People’s Republic of Korea.

Security experts in recent years have begun to shift their focus away from disarming the country to studying methods for deterring the country’s desire to use or share nuclear weapons.

At the same time, the North has made steady progress in its land- and sea-based missile programs, which already have the ability to strike regional American allies in Seoul or Tokyo. In a televised New Year’s Day message this year, Kim boasted that the country was also making significant progress in its effort to launch intercontinental ballistic missiles capable of striking American targets in the Pacific Ocean, or perhaps even the U.S. mainland.

Kim’s New Year’s address pushed then President-elect Trump to tweet: “North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S. It won’t happen!�

Trump’s administration is still adapting to its new policy of pressure and engagement on North Korea. Such efforts toward North Korea have baffled the last three American presidents who watched, with few good options for intervention, as the country became a nuclear state.

In a visit to Seoul in March, Secretary of State Rex Tillerson called for a “different approach� for dealing with the North’s nuclear ambitions, acknowledging that previous administrations’ efforts to apply pressure and use covert actions have failed. It’s unclear what that approach might be, however, though Tillerson did suggest that military intervention was still an option.

A looming concern for American officials is the extent to which China can — or is willing to — apply additional economic pressure to persuade the North to denuclearize, or perhaps to talk about it. Trump has said that the United States would tackle the problem alone, if needed, a posture questioned by experts who note the issue’s regional complexity.

Some in South Korea, whose densely populated capital is within striking range of conventional weapons like artillery, see the recent provocations as a test for China. Its leaders, including President Xi Jinping, have urged restraint.

Last month, members of the United Nations Security Council, including China, voted to impose a new round of sanctions targeting North Korea’s primary exports, including iron and coal, in retaliation for missile tests. North Korea said the United States will “pay dearly� for the sanctions, estimated to be worth about $1 billion to the North Korean economy.

Stiles is a special correspondent. Times staff writer Jonathan Kaiman in Beijing contributed to this report.

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UPDATES:

6:15 a.m.: Updated with President Trump’s tweet.

12:50 a.m.: Updated with reaction in China.

12:05 a.m., Sept. 3: Updated to report that North Korea confirms it detonated a nuclear device.

10:47 p.m.: Updated with comments from Harry Kazianis and details on economic sanctions imposed on North Korea last month.

10 p.m.: Updated with background on U.S.-North Korean relations and additional details on the apparent nuclear test.

9:15 p.m.: Updated with the seismic wave.

This article was first published at 5:10 p.m.