SEOUL, South Korea (AP) — North Korea said it conducted a powerful hydrogen bomb test Wednesday, a defiant and surprising move that, if confirmed, would be a huge jump in Pyongyang's quest to improve its still-limited nuclear arsenal.

South Korea's spy agency and outside nuclear experts cast strong doubt, however, saying the estimated explosive yield from North Korea's fourth nuclear explosion was much smaller than what even a failed H-bomb detonation would produce.

The doubts didn't stop jubilation and pride in Pyongyang. A North Korean television anchor, reading a typically propaganda-heavy statement, said a test of a "miniaturized" hydrogen bomb had been a "perfect success" that elevated the country's "nuclear might to the next level." State media later crowed that its "H-bomb of justice" lets it stand firm against U.S. aggression.

A large crowd celebrated in front of Pyongyang's main train station as the announcement was read on a big video screen, with people taking videos or photos of the screen on their mobile phones and applauding and cheering.

In Seoul and elsewhere there was high-level worry. South Korean President Park Geun-hye ordered her military to bolster its combined defense posture with U.S. forces and called the test a "grave provocation" and "an act that threatens our lives and future." Japanese Prime Minister Shinzo Abe said, "We absolutely cannot allow this."

Washington and nuclear experts have been skeptical about past North Korean claims about H-bombs, which are much more powerful and much more difficult to make, than atomic bombs. A confirmed test would further worsen already abysmal relations between Pyongyang and its neighbors and lead to a strong push for tougher sanctions on North Korea at the United Nations. The Security Council quickly announced an emergency meeting.

Whatever the type of the test, North Korea's fourth nuclear explosion will likely push Pyongyang's scientists and engineers closer to their goal of building a bomb small enough to place on a missile that can reach the U.S. mainland.

A successful H-bomb test would be a big new step for the North. Fusion is the main principle behind the hydrogen bomb, which can be hundreds of times more powerful than atomic bombs that use fission. In a hydrogen bomb, radiation from a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.

A South Korean lawmaker said the country's spy agency told him in a private briefing that Pyongyang may not have conducted an H-bomb test given the relatively small size of the seismic wave reported.

An estimated explosive yield of 6.0 kilotons and a quake with a magnitude of 4.8 (the U.S. reported 5.1) were detected, lawmaker Lee Cheol Woo said the National Intelligence Service told him. That's smaller than the estimated explosive yield of 7.9 kilotons and a quake with a magnitude of 4.9 that were reported after the 2013 nuclear test, he said, and only a fraction of the hundreds of kilotons that a successful H-bomb test's explosion would usually yield. Even a failed H-bomb detonation typically yields tens of kilotons, the NIS told Lee, who sits on the parliament's intelligence committee.

A miniaturized H-bomb can trigger a weak quake magnitude, but only the U.S. and Russia have such H-bombs, Lee cited the NIS as saying.

While also noting the quake magnitude was likely too small for an H-bomb test, Jaiki Lee, a professor of nuclear engineering at Seoul's Hanyang University, said the North could have experimented with a "boosted" hybrid bomb that uses some nuclear fusion fuel along with more conventional uranium or plutonium fuel.

After North Korean leader Kim Jong Un bragged of H-bomb capabilities in December, nuclear expert Jeffrey Lewis also questioned Pyongyang's ability to build such a bomb.

But, he wrote on the North Korea-focused 38 North website, "The North has now had a nuclear weapons program for more than 20 years. This program has yielded three nuclear tests. North Korean nuclear scientists have access to their counterparts in Pakistan, possibly Iran and maybe a few other places. We should not expect that they will test the same fission device over and over again."

In Pyongyang, meanwhile, the announcement was greeted with an expected rush of nationalistic pride, and some bewilderment.

Kim Sok Chol, 32, told The Associated Press that he doesn't know much about H-bombs, but added that "Since we have it the U.S. will not attack us."

University student Ri Sol Yong, 22, said, "If we didn't have powerful nuclear weapons, we would already have been turned into the slaves of the U.S."

It could take weeks before the true nature of the test is confirmed by outside experts — if they are able to do so at all. North Korea goes to great lengths to conceal its tests by conducting them underground and tightly sealing off tunnels or any other vents though which radioactive residue and blast-related noble gases could escape into the atmosphere.

The U.S. Air Force has aircraft designed to detect the evidence of a nuclear test, and such aircraft could be deployed from a U.S. base on the Japanese island of Okinawa to search for clues. Japanese media said Tokyo has also mobilized its own reconnaissance aircraft for sorties over the Sea of Japan to try to collect atmospheric data.

North Korea's previous nuclear test was in early 2013, and Kim Jong Un did not mention nuclear weapons in his annual New Year's speech. Some outside analysts speculated Kim was worried about deteriorating ties with China, the North's last major ally, which has shown greater frustration at provocations and a possible willingness to allow stronger U.N. sanctions.

Chinese Foreign Ministry spokeswoman Hua Chunying told reporters that Beijing "firmly opposes" Pyongyang's purported test and is monitoring the environment on its border with North Korea near the test site.

Just how big a threat North Korea's nuclear program currently poses is something of a mystery. North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to eventually carry smaller versions of those bombs.

Some analysts say the North hasn't likely achieved the technology needed to manufacture a miniaturized warhead that could fit on a long-range missile capable of hitting the U.S. mainland. But there is a growing debate on just how far the North has advanced in its secretive nuclear and missile programs.

North Korea needs nuclear tests for practical military and political reasons. To build a credible nuclear program, the North must explode new nuclear devices — including miniaturized ones — so its scientists can continually improve their designs and technology. Nuclear-tipped missiles could then be used as deterrents, and diplomatic bargaining chips, against its enemies — and especially against the United States, which Pyongyang has long pushed to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War.

"This is indeed a wakeup call," Lassina Zerbo, the head of the Vienna-based U.N. Comprehensive Test Ban Treaty Organization, which has a worldwide network of monitoring stations to detect nuclear testing, told AP by phone. "I am convinced it will have repercussions on North Korea and international peace and stability."

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AP correspondent Hyung-jin Kim and writer Kim Tong-hyung in Seoul, Pyongyang bureau chief Eric Talmadge in Tokyo and writer George Jahn in Vienna contributed to this story.

Follow Foster Klug, Seoul's bureau chief, on Twitter: @APKlug