SEOUL—North Korea said it successfully staged its first test of a more powerful form of nuclear weapon, expanding the U.S.’s foreign-policy challenges and highlighting the limits of China’s ability to rein in its volatile ally.
North Korean state television said in a midday broadcast that scientists had successfully detonated a hydrogen bomb at around 10 a.m. local time.
The U.S. Geological Survey reported that it detected a magnitude 5.1 earthquake at that time near North Korea’s nuclear test site in the country’s northeast.
Experts have said it was unclear whether North Korea had developed the ability to build a hydrogen bomb. The magnitude of the latest explosion was the same as a 2013 test of an atomic bomb.
State Department spokesman John Kirby said the U.S. couldn’t confirm North Korea’s claims of a nuclear test but is monitoring the situation.
“We are aware of seismic activity on the Korean Peninsula in the vicinity of a known North Korean nuclear test site and have seen Pyongyang’s claims of a nuclear test,” Mr. Kirby said.
South Korea denounced the test as a violation of United Nations Security Council resolutions that ban nuclear-weapons development by the North.
“This is a grave provocation to our security, threatening the survival and future of our nation and further directly challenging peace and stability in the world,” South Korean President Park Geun-hye said during a meeting of the National Security Council in Seoul. “The government—in close cooperation with the international community—should have North Korea pay a price without fail for the latest nuclear test.” She urged “powerful sanctions” by the U.N. Security Council and international community, as well as “resolute measures” by the U.S. and other allies.
Japan, which is serving on the U.N. Security Council as a nonpermanent member, will work closely with countries including the U.S., South Korea, China and Russia, Prime Minister Shinzo Abe said, adding that his government won’t tolerate this “grave threat” to Japan’s security. France, Russia and the U.K. also said if the device was detonated as described, it would be a violation of U.N. Security Council resolutions. The European Union’s foreign policy chief Federica Mogherini called on North Korea to “cease this illegal and dangerous behavior.”
At the U.N., U.S. and Japanese officials said early Wednesday that they have asked for an emergency meeting of the U.N. Security Council to discuss North Korea. Following Pyongyang’s nuclear test in 2013, the U.N. Security Council adopted sanctions designed to tighten financial restrictions on North Korea in a resolution that won the support of North Korean ally China.
At a regular press briefing on Wednesday, foreign ministry spokesperson Hua Chunying said China knew nothing about the test before it was carried out. Asked whether sanctions would come from Beijing, Ms. Hua said China would “honor its international obligations to realize the denuclearization of the peninsula.”
For China, the test will strain relations with North Korea and increase international pressure to participate more in efforts to control it, said Korea expert Cai Jian, of Fudan University in Shanghai.
“China would have to not only worry about the stability in its border areas but also about if the U.S. and Japan would use chances like this as excuses to deploy more military facilities in the area,” he added.
In the past, U.S. officials have questioned North Korea’s claims to technological breakthroughs, though last year the Pentagon did say it had determined that Pyongyang was close to or already capable of miniaturizing nuclear warheads so they can be mounted on missiles and launched across the globe.
Confirmation that Pyongyang has conducted a nuclear test of any kind is likely to accelerate U.S. responses across a variety of fronts, and add to pressure on the Obama administration to step up action on sanctions and missile defense.
While the nature of the latest test remained unclear, it highlighted the commitment of North Korea’s young leader, Kim Jong Un, to continue confronting its neighbors despite recent hopes in South Korea of a shift in Pyongyang. Following an armed standoff in August, the two Koreas agreed to hold high-level dialogue, but the talks stalled due to the two side’s differing priorities.
North Korea has previously staged three test detonations of atomic bombs; the latest was in 2013.
A hydrogen bomb – technically known as a “thermonuclear weapon” – usually uses a smaller, primary atomic explosion to ignite a secondary, much larger blast. The first stage is based on nuclear fission — the splitting of atoms — and the second on nuclear fusion, which combines atoms, smashing them together and unleashing more energy. Additional stages can be added to increase its destructive force.
That makes the H-bomb far more powerful than early nuclear weapons that typically used a single-stage blast based only on nuclear fission. Those weapons are known as “pure fission” devices and are thought to have been used in all of North Korea’s three previous nuclear tests, which it said involved atomic bombs.
However, “The yield of this explosion doesn’t seem to be big enough to be a classic hydrogen bomb,” said Jeffrey Lewis, director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies.
In a statement from its state media following the test, North Korea said its scientists had verified “the power of a smaller H-bomb.”
U.S. officials say they have repeatedly tried to engage North Korea in dialogue about its nuclear program in recent months, but Pyongyang hasn’t responded to their advances.
The U.S. has leaned heavily on China in its bid to rein in North Korea. President Barack Obama hosted Chinese President Xi Jinping for a state visit in September at which the White House pressed for the two to lean in unison on Pyongyang. A new test now would prompt a redoubling of U.S. pressure on Beijing.
Coming in the midst of a highly charged U.S. presidential campaign in which foreign policy is a hot issue, the North Korean test is also likely to fuel charges that Mr. Obama has been lenient with U.S. adversaries.
Sen. Marco Rubio (R., Fla.) became the first Republican primary hopeful to react, saying Pyongyang’s claim highlighted the flaws of the Obama administration, in which Democratic presidential hopeful Hillary Clinton served as secretary of State.
“If this test is confirmed, it will be just the latest example of the failed Obama-Clinton foreign policy,” he said in a Twitter message.
A 2014 cyberattack against Sony Pictures Entertainment was traced by U.S. officials to North Korea, and Mr. Obama blamed Pyongyang for the hack, threatening a response.
But the U.S. didn’t take threatened actions, such as placing North Korea back on the U.S. list of countries that sponsor terrorism, a move that was widely supported in Congress and would have instituted new sanctions.
Republicans have continuously pushed the Obama administration to beef up its commitment to missile defense both on the mainland and abroad.
The U.S. government authorized $7.87 billion in spending by the Missile Defense Agency last year, down from a peak of $9.4 billion in fiscal year 2007, according to agency records.
Mr. Obama has called North Korea the most isolated nation on earth and doubted whether new sanctions would change its behavior. But his critics said Tuesday that a tough U.S. reaction is required.
“The U.S. must lead with a firm response, including an enhancement of sanctions that targets the financial resources of the regime that fund its misrule and nuclear proliferation,” said Sen. Tom Cotton (R., Ark.). “The world is more dangerous when the United States is neither trusted nor feared.”
U.S. and South Korean officials have been discussing joint efforts to defend against a possible missile assault, but haven’t agreed on using an advanced U.S. missile system known as the Terminal High Altitude Area Defense, or THAAD.
U.S. officials have suggested a THAAD system in South Korea, but Seoul has questioned whether it is the right system for them, and noted possible concerns in China or Russia about development of a U.S. missile system.
John Delury, a professor at Yonsei University in Seoul, said the decision to go ahead with a test was likely driven by technical needs to continue to test nuclear devices.
In a speech on New Year’s Day, Mr. Kim said he remained committed to a military confrontation with other nations, even as he called for talks with South Korea. Following an armed standoff in August, the two Koreas agreed to hold high-level dialogue but the talks have stalled due to differing priorities held by each side.
—Damian Paletta and Jay Solomon in Washington, D.C., Te-Ping Chen and Yang Jie in Beijing and Jun Hongo in Tokyo contributed to this article.