SEOUL, Korea, Republic Of -- North Korea's declaration that it had tested a hydrogen bomb for the first time was greeted with widespread condemnation -- but also skepticism -- as world powers vowed Wednesday to punish the impoverished and defiant nation with new international sanctions.

The isolated country's fourth nuclear test since 2006 was a "reckless challenge to international norms of behaviour and the authority of the U.N. Security Council," said British Ambassador Matthew Rycroft.

The council met in an emergency session and called the test "a clear violation" of its resolutions. It agreed to start work immediately on a resolution for new sanctions.

The international community must respond with "steadily increasing pressure" and rigorous enforcement of existing measures, said U.S. Ambassador Samantha Power.

Four rounds of U.N. sanctions have aimed at reining in the North's nuclear and missile development, but Pyongyang has ignored them and moved ahead with programs to modernize its ballistic missiles and nuclear weapons.

The last sanctions resolution in 2013 was co-sponsored by the U.S. and China, and both countries will be key to an agreement on a new one. Whether any new sanctions can slow North Korea's nuclear program, however, remains to be seen.

There was a burst of jubilation and pride in North Korea's capital of Pyongyang, where a TV anchor said Wednesday's test of a "miniaturized" hydrogen bomb had been a "perfect success" that elevated the country's "nuclear might to the next level."

A successful test would mark a major and unanticipated advance for the North's still-limited nuclear arsenal and push 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 an early analysis by the U.S. government was "not consistent with the claims that the regime has made of a successful hydrogen bomb test," White House spokesman Josh Earnest said.

He 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.

"We're trying to run down their assertion," U.S. Vice-President Joe Biden told a local TV station in Virginia. "We are determining whether the claim is accurate."

Hours earlier, South Korea's spy agency said it thought the estimated explosive yield from the blast was much smaller than what even a failed hydrogen bomb detonation would produce.

South Korean lawmaker Lee Cheol Woo said he was told in a briefing by the National Intelligence Service 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, Lee said he was told. That's smaller than the estimated 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."

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, a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.

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 neighbours and lead to a strong push for tougher sanctions.

Despite the doubt, there was still high-level concern in Seoul and elsewhere.

South Korean President Park Geun-hye ordered her military to bolster its combined defence 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.N. Secretary-General Ban Ki-moon, a former South Korean foreign minister, called North Korea's action "profoundly destabilizing for regional security," and he demanded that Pyongyang halt any further nuclear activities.

U.S. Defence Secretary Ash Carter spoke by phone with his South Korean counterpart Han Min-Koo, and they agreed 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.

The test was unexpected in part because North Korea's last one was nearly three years ago and its leader, 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" such a test and was monitoring the environment on its border with North Korea near the northeastern region where Pyongyang has conducted its nuclear tests.

North Korea's state media called the test a self-defence 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-defence and a very just step no one can slander."

A large crowd celebrated in front of Pyongyang'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. There was 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."

The hydrogen bomb already is the global standard for the five nations with the greatest nuclear capabilities: the U.S., Russia, France, the U.K. and China. Other nations may also either have it or are working on it, despite a worldwide effort to contain such proliferation.

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 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.

To build its 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 -- 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 worldwide monitoring stations to detect nuclear tests. "I am convinced it will have repercussions on North Korea and international peace and stability."

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. aircraft designed to detect 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 over the Sea of Japan to try to collect atmospheric data.

Zerbo said its monitors are looking for confirmation by detecting radioisotopes, which is the only way to determine if it was indeed a hydrogen bomb or similar to previous North Korean tests. It took over 50 days to detect radioisotopes venting from the last test in 2013, he added.

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

Ferenc Dalnoki-Veress, a physicist 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, "but maybe we'll be lucky."

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