SEOUL, South Korea >> 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.

A television anchor said in a typically propaganda-heavy statement that the North had tested a “miniaturized” hydrogen bomb, elevating the country’s “nuclear might to the next level” and providing it with a weapon to defend against the United States and its other enemies.

The statement said the test was a “perfect success,” and the announcement was celebrated on the streets of Pyongyang, where a large crowd gathered in front of the city’s main train station to watch the test announcement on a big video screen. Some people took videos or photos of the screen on their mobile phones; others applauded and cheered.

In Seoul and elsewhere there was high-level worry, and also some doubt among experts. 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.”

There has long been skepticism by Washington and nuclear experts 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, however, would be seen as extremely worrying and lead to a strong push for new, tougher sanctions on North Korea at the United Nations. The Security Council, which is responsible for international sanctions, quickly announced an emergency meeting on North Korea. It would also further worsen already abysmal relations between Pyongyang and its neighbors.

North Korean nuclear tests capture the world’s attention because each new blast is seen as pushing North Korea’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, and the announcement prompted skepticism. Fusion is the main principle behind the hydrogen bomb, which can be hundreds of times more powerful than atomic weapons 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.

Writing in December, after North Korean leader Kim Jong Un bragged of H-bomb capabilities, nuclear expert Jeffrey Lewis noted that building such a bomb “would seem to be a bit of a stretch for the North Koreans.”

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.”

One expert in Seoul said the reported 5.1 magnitude of the quake set off by Wednesday’s test was likely too small to be an H-bomb test. However, the North could have experimented with a “boosted” bomb that uses some nuclear fusion fuel along with more conventional uranium or plutonium fuel, said Jaiki Lee, a professor of nuclear engineering at Seoul’s Hanyang University.

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, according to Yonhap news agency.

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 residues and blast-related noble gases could escape into the atmosphere.

The United States 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.

Just how big a threat North Korea’s nuclear program currently poses to the outside world 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 UN 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.”

An official from the Korea Metrological Administration, South Korea’s weather agency, said the agency believed the 5.1 magnitude earthquake was caused artificially, based on an analysis of the seismic waves; it originated 49 kilometers (30 miles) north of Kilju, the northeastern area where North Korea’s main nuclear test site is located. The country conducted all three previous atomic detonations there.

The announcement came as a surprise.

North Korea hadn’t conducted a nuclear explosion since early 2013, and leader Kim Jong Un did not mention the country’s nuclear weapons in his New Year’s speech. Some outside analysts speculated that Kim was worried about deteriorating ties with China, the North’s last major ally, which has shown signs of greater frustration at provocations and a possible willingness to allow stronger U.N. sanctions.

Pyongyang says its nuclear weapons program is necessary to defend itself against the United States. North Korea under Kim Jong Un has pledged to bolster its nuclear arsenal unless Washington scraps what Pyongyang calls a hostile policy.

The Korean Peninsula remains technically at war, as the 1950-53 conflict ended in an armistice. Washington stations more than 28,000 troops in South Korea as a buttress against any North Korean aggression. Tens of thousands more are in nearby Japan.

Six-nation negotiations on dismantling North Korea’s nuclear program in exchange for aid were last held in late 2008 and fell apart in early 2009, when North Korea was led by Kim Jong Un’s father, Kim Jong Il, who died in late 2011.

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AP writer Hyung-jin Kim in Seoul, Eric Talmadge in Tokyo and George Jahn in Vienna contributed to this story.