SEOUL » Soon after the ground shook around its nuclear testing facility, North Korea trumpeted its first hydrogen bomb test — a self-proclaimed “H-bomb of justice” that would mark a major and unanticipated advance for its still-limited nuclear arsenal.

The announcement today by the defiant, impoverished country was met with widespread skepticism, as well as a strong condemnation by the U.N. Security Council, which said it would begin work on a resolution for new international sanctions.

North Korea’s fourth nuclear test likely pushed its scientists and engineers closer to their goal of building a warhead small enough to place on a missile that can reach the U.S. mainland. But South Korea’s spy agency thought the estimated explosive yield from the blast was much smaller than what even a failed hydrogen bomb detonation would produce, and the White House said its early analysis of underground activity “is not consistent” with the North’s claim of a successful H-bomb test.

There was a burst of jubilation and pride in Pyongyang. A North Korean TV anchor said the test of a “miniaturized” hydrogen bomb had been a “perfect success” that elevated the country’s “nuclear might to the next level.”

A large crowd celebrated in front of the capital’s main train station as the announcement was read on a big video screen, with people applauding, cheering and recording the report on their mobile phones.

North Korea’s state media called the test a self-defense measure against a potential U.S. attack. “The (country’s) access to H-bomb of justice, standing against the U.S., the chieftain of aggression …, is the legitimate right of a sovereign state for self-defense and a very just step no one can slander.”

There was high-level concern in Seoul and elsewhere. South Korean President Park Geun-hye ordered her military to bolster its combined defense posture with U.S. forces. She 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.”

U.S. Defense Secretary Ash Carter spoke by phone with his South Korean counterpart Han Min-Koo, and they agreed that a North Korean nuclear test would be an “unacceptable and irresponsible provocation,” according to Carter’s spokesman, Peter Cook.

Cook said Carter reaffirmed the U.S. treaty commitment to defend South Korea, which he said includes “all aspects of the United States’ extended deterrence” — an allusion to a longstanding U.S. promise to defend South Korea with nuclear weapons if necessary.

In saying an early analysis by the U.S. was “not consistent with the claims that the regime has made of a successful hydrogen bomb test,” White House spokesman Josh Earnest added that nothing has happened in the last 24 hours to change Washington’s assessment of Pyongyang’s technical or military capabilities.

The U.S. is still doing the work needed to learn more about the North’s test, he added.

Washington and nuclear experts have been skeptical of past North Korean claims about hydrogen 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.

Following a closed-door, emergency session, the U.N. Security Council called the test “a clear violation” of the council’s resolutions.

“Therefore a clear threat to international peace and security continues to exist,” the statement said.

The council said it would begin work immediately on a new sanctions resolution in light of “the gravity of this violation.”

Before the meeting, U.N. Secretary-General Ban Ki-moon condemned the test as “profoundly destabilizing for regional security,” and demanded North Korea cease further nuclear activities and meet its obligations for “for verifiable denuclearization.”

A successful H-bomb test would be a big advance in North Korea’s nuclear weapons program. 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 briefing that Pyongyang may not have conducted a hydrogen 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 he was told by the National Intelligence Service. That’s smaller than the estimated explosive yield of 7.9 kilotons and 4.9-magnitude quake reported after a 2013 nuclear test, he said, and only a fraction of the hundreds of kilotons that a successful H-bomb test 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, but only the U.S. and Russia have such weapons, Lee cited the NIS as saying.

“I’m pretty skeptical,” said Melissa Hanham, senior researcher at the James Martin Center for Nonproliferation Studies at the Middlebury Institute for International Studies in Monterey, California. “The seismic data indicates it would be very small for a hydrogen test.

“It seems just too soon to have this big technical achievement,” she said. “But North Korea has always defied expectations.”

While also noting the quake 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.

Joel Wit, founder of the North Korea-focused 38 North website, said a boosted bomb “is the most likely option,” while adding that he isn’t surprised that North Korea has shifted focus to hydrogen weaponry.

“Every nuclear power essentially moves down the same track as they develop nuclear weapons,” he said. “And that track is miniaturization, but also increasing the yield of nuclear weapons. That’s what the Americans did; that’s what the Russians did.”

The announcement was greeted in Pyongyang 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: “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 be weeks before the true nature of the test is confirmed by outside experts — if they are able to do so at all.

U.S. Air Force aircraft designed to detect the evidence of a nuclear test, such as radioactive particulate matter and blast-related noble gases, could be deployed from a U.S. base on the Japanese island of Okinawa. Japanese media said Tokyo mobilized its own reconnaissance aircraft for sorties over the Sea of Japan to try to collect atmospheric data.

But North Korea goes to great lengths to conceal its tests by conducting them underground and tightly sealing off tunnels or other vents through which radioactive residue could escape.

Ferenc Dalnoki-Veress, a physicist, scientist-in-residence and professor at the James Martin Center, said it may not be possible for the monitors to determine if the explosion was caused by a hydrogen bomb.

“For that, you might need to have the particulates,” he said. “But maybe we’ll be lucky.”

The test was unexpected in part because North Korea’s last nuclear test was nearly three years ago and Kim Jong Un did not mention nuclear weapons in his annual New Year’s speech. Some outside analysts had 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 said 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 poses is 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 probably hasn’t achieved the technology needed to make a miniaturized warhead that could fit on a long-range missile capable of hitting the U.S. mainland. But debate is growing on just how far the North has advanced.

North Korea needs fresh nuclear tests for practical military and political reasons. To build a credible nuclear program, the North must explode new and more advanced devices so scientists can 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 U.S., 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,” said Lassina Zerbo, head of the Vienna-based U.N. Comprehensive Test Ban Treaty Organization, which has a worldwide network of monitoring stations to detect nuclear tests. “I am convinced it will have repercussions on North Korea and international peace and stability.”

AP Pyongyang Bureau Chief Eric Talmadge in Tokyo, Hyung-jin Kim and Kim Tong-hyung in Seoul, Robert Burns and Josh Lederman in Washington, Edith M. Lederer and Cara Anna at the United Nations, George Jahn in Vienna and Tim Sullivan in New Delhi contributed to this story.