Updated Sept. 9, 2016 5:40 a.m. ET
SEOUL—North Korea conducted a fifth nuclear test hours after President Barack Obama wrapped up a tour of Asia, highlighting the U.S.’s struggle to rein in the rising threat from dictator Kim Jong Un.
Pyongyang declared a successful test hours after the U.S. Geological Survey detected a magnitude 5.3 earthquake near North Korea’s nuclear test site in the country’s northeast early on Friday, a reading that surpassed the magnitudes of tremors set off by the country’s previous nuclear tests.
North Korea confirmed in a statement released through its state media that it conducted a test explosion of a nuclear warhead. It said the test was successful and confirmed its ability to produce nuclear-tipped missiles “at will.” It added that it would continue to build up its nuclear force in quality and quantity.
Ryoo Yong-gyu, a director of earthquake and volcano monitoring in South Korea, spoke in Seoul about seismic activity on the Korean peninsula on Friday. The 5.3-magnitude earthquake in North Korea suggests that the country is testing increasingly powerful nuclear devices. Credit Jeon Heon-Kyun/European Pressphoto Agency
China condemned North Korea’s nuclear test, urging Pyongyang to “honor its commitment to denuclearization, comply with relevant [United Nations] Security Council resolutions and stop taking any action to worsen the situation,” according to the Foreign Ministry.
Japanese Prime Minister Shinzo Abe called North Korea’s nuclear test a “grave threat” to Japan and held a phone meeting with Mr. Obama to discuss how to respond.
“The international community needs to deal with North Korea firmly and make Pyongyang understand the costs of taking such provocative action,” Mr. Abe told Mr. Obama in the 10-minute conversation, according to the Japanese Foreign Ministry.
Mr. Obama, who was flying back to the U.S. from Laos, reiterated Washington’s security commitment to Japan, including its extended nuclear deterrence, or willingness to use nuclear weapons to protect its allies, the foreign ministry said.
The explosion, on the 68th anniversary of North Korea’s founding as a state, came a day after Mr. Obama said at a regional summit in Laos that he would continue to explore ways to reduce the threat from North Korea during his final four months in office.
After North Korea fired three ballistic missiles this week, Mr. Obama called for tightening international sanctions and closing loopholes that he said prevent them from being as effective as intended. “We are deeply disturbed by what’s happened,” Mr. Obama said. He also said China—North Korea’s sole major ally and trading partner—needed to work “more effectively” with the U.S. to curtail the North Korea threat.
The White House sees North Korea as among the most serious challenges facing Mr. Obama’s successor in January. Officials have said they don’t expect to make any significant progress on the North Korea threat, which has worsened during Mr. Obama’s two terms in office.
Friday’s underground test underscores a significant weakness in Mr. Obama’s so-called Asia pivot and raises the stakes for his final appearance as president at the United Nations General Assembly this month. The U.N. gathering of world leaders is poised to become the platform on which U.S. officials scramble to corral a swift response to Pyongyang.
At the U.N. gathering, Mr. Obama will also push to strengthen global nuclear nonproliferation measures. U.S. officials are considering pressing other countries for closer international adherence to a ban on nuclear tests.
In January, North Korea detonated a nuclear device at the same site, which it called its first successful test of a new, more powerful type of thermonuclear weapon that uses hydrogen.
But experts estimated that detonation to have the power of around seven kilotons of TNT, which they said was too small for a hydrogen bomb. It was similar to three other less-powerful atomic bombs North Korea has tested since 2006. The January detonation triggered a magnitude 5.1 quake.
The latest explosion also appeared too small to be caused by a thermonuclear device, said Jeffrey Lewis, a director at the James Martin Center for Nonproliferation Studies in Monterey, Calif. Mr. Lewis said it may have instead been a boosted fission device that uses some hydrogen to make a traditional nuclear bomb more powerful.
North Korea’s nuclear detonations since 2006 have all been estimated to be less than 10 kilotons of TNT. South Korea’s Defense Ministry said its initial analysis was that Friday’s explosion was caused by a nuclear device with a force equivalent to 10 kilotons of TNT.
By contrast, the first nuclear bomb tested by the U.S. in 1945 had an explosive force of about 18 to 20 kilotons.
Since January, North Korean leader Kim Jong Un has called for further nuclear and missile tests as Pyongyang elevated its threatening rhetoric and made a series of claims about being able to hit South Korea, Japan and U.S. bases in Asia with nuclear-tipped missiles.
Recent successful tests of missiles launched from a mobile carrier and a submarine show North Korea is gaining the capacity to threaten its enemies even if its military bases are destroyed. Some experts say Pyongyang now appears to be only two or three years away from proving the ability to hit the continental U.S. with a nuclear-tipped missile.
South Korea has responded to North Korea’s missile progress by announcing a decision to deploy a U.S.-built advance missile defense shield by the end of next year.
“The real issue is whether North Korea can marry a nuclear device to a missile,” said Robert Kelly, a professor of political science at Pusan National University. “The nuclear tests are unwanted of course, but not that destabilizing because we’ve known for a while that North Korea has functional nuclear weapons.”
The earthquake occurred at 9.30 a.m. local time about 230 miles northeast of Pyongyang, the U.S. Geological Survey said.
The reaction from other world leaders was similarly swift.
South Korean President Park Geun-hye said: “Through this nuclear test, the Kim Jong Un regime will face even stronger international sanctions and isolation, hastening its self-destruction.”
French President François Hollande “strongly condemned” North Korea’s nuclear test and called on the international community to unite against provocation from Pyongyang.
Following North Korea’s nuclear test in January and a long-range rocket launch in February, the U.N. imposed stricter sanctions on Pyongyang designed to disrupt its international trade. The U.S. has also imposed further bilateral sanctions on North Korea and blacklisted Mr. Kim.
A U.N. spokesman on Friday said the organization would issue an official comment on Saturday.
Earlier this week, the Security Council said it was discussing new punitive measures against North Korea in response to its ballistic missile launch that landed near the coast of Japan on Monday. The Council is scheduled to meet Friday and North Korea will likely dominate the discussion, with U.S. and allies pushing for immediate sanctions.
—Farnaz Fassihi at the United Nations, Mitsuru Obe in Tokyo and Te-Ping Chen in Beijing contributed to this article.
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North Korea Claims Success in Latest Nuclear Test
SEOUL, South Korea — North Korea conducted its fifth underground nuclear test on Friday, its government said, despite threats of more sanctions from the United States and the United Nations. The latest test, according to South Korean officials, produced a more powerful explosive yield than the North’s previous detonations, indicating that the country was making progress in its efforts to build a functional nuclear warhead.
The test confirmed the explosive power and other characteristics of a “nuclear warhead that has been standardized to be able to be mounted on” its ballistic missiles, the North’s nuclear weapons institute said in a statement on Friday.
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