Hours after North Korea trumpeted the apparent "perfect success" of its first hydrogen bomb test, the United Nations Security Council fired back with a promise of new sanctions.

Reading from a statement drafted by the 15-member council, Uruguay's UN Ambassador Elbio Rosselli said the Security Council would "immediately" begin work on new measures.

"A clear threat to international peace and security continues to exist," the statement read.

Pyongyang's announcement Wednesday about the test — which would mark a major and unanticipated advance for North Korea's still-limited nuclear arsenal — was met with widespread skepticism. But whatever the North detonated in its fourth nuclear test may have pushed Pyongyang's scientists and engineers closer to their goal of building a warhead small enough to place on a missile that could reach the U.S. mainland.

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South Korea's spy agency thought the estimated explosive yield from the explosion was much smaller than what even a failed H-bomb detonation would produce.

The test was met with a burst of 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."

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People in Pyongyang watch a huge screen broadcasting the North Korean government's announcement of a 'miniaturized' hydrogen bomb test, which officials called a 'perfect success.' Outside experts have been skeptical about North Korean claims about H-bombs, which are much more difficult to make than atomic bombs. (Kyodo/Reuters)

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 defence 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."

China, the North's closest ally, condemned the test, which was staged close enough to the border to send palpable tremors into northeastern China, prompting schools to be evacuated.

The political reverberations in Beijing will likely be just as dramatic, boding ill for a relationship already under strain.

"China firmly opposes this nuclear bomb test by North Korea," Foreign Ministry spokeswoman Hua Chunying told reporters at a daily briefing. "North Korea should stop taking any actions which would worsen the situation on the Korean Peninsula."

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North Korean leader Kim Jong-un said last month in a rare public appearance that the country had developed a hydrogen bomb. (Wong Maye-E/Associated Press)

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 neighbours and lead to a strong push for tougher sanctions on North Korea at the United Nations.

Speaking just before Wednesday's closed-door Security Council meeting, UN Secretary General Ban Ki-moon called the H-bomb news "deeply troubling" and demanded North Korea cease such activities immediately.

"This act is profoundly destabilizing for regional security and seriously undermines international non-proliferation efforts," he said. "I condemn it unequivocally."

Seismic activity reported

A successful H-bomb test would be a big advance. 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.

North Korea quake map

This intensity map created by the U.S. Geological Survey shows the origin of an earthquake in North Korea, along with locations of where the seismic activity was felt. (U.S. Geological Survey)

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

South Korea's meteorological agency said separately that it had not detected any radiation after the test.

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.

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."

'The U.S. will not attack us'

In Pyongyang, 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."

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People watch a news report on North Korea's first hydrogen bomb test at a railroad station in Seoul on Wednesday. The image shown on TV shows files images from other nuclear tests from other countries. (Jung Yeon-Je/AFP/Getty Images)

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 through which radioactive residue and blast-related noble gases could escape into the atmosphere.

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

The test was unexpected in part because 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 UN sanctions.

Threat of the North's program still a mystery

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 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 fresh 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."

With files from Reuters