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SEOUL—North Korea said Wednesday that it successfully staged its first test of a more powerful form of a nuclear weapon, expanding the U.S.’s foreign-policy challenges and highlighting the limits of China’s ability to rein in its volatile ally. An announcer on 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 a magnitude 5.1 earthquake was triggered at that time near North Korea’s nuclear test site in the northeast of the country. 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. U.S. officials said they were working to confirm North Korea’s claim. In the past, U.S. officials have questioned North Korea’s claims to technological breakthroughs. At the same time, the Pentagon last year said it has determined that Pyongyang is 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. North Korea has staged three test detonations of atomic bombs; the latest was in 2013. A hydrogen bomb is more powerful and uses nuclear fusion to trigger an uncontrolled chain reaction. “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.” South Korea denounced the test as a violation of United Nations Security Council resolutions that ban the North’s nuclear weapons development. “North Korea’s provocation is in clear violation of Security Council resolutions and a serious challenge to international peace and security,” Vice Foreign Minister Lim Sung-nam said during an emergency meeting of ministry officials.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. 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. 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 of a shift in its emphasis on dialogue. 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.