SEOUL, South Korea (AP) -- North Korea said Wednesday it had conducted a powerful hydrogen bomb test, 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." South Korea's Defense Ministry said, without elaborating, that it was bolstering its security posture in response.
There has long been skepticism by Washington and nuclear experts on past North Korean claims about H-bombs, which are much more powerful, and much more difficult to make, than atomic bombs. But a confirmed test would be seen as extremely worrying and lead to a strong push for new, tougher sanctions on North Korea at the United Nations. It would also further worsen already abysmal relations between Pyongyang and its neighbors.
North Korean nuclear tests are worrying 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 step for the North. 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."
"Thermonuclear weapons are tricky; making one work requires a bit of test experience," he wrote on the North Korea-focused 38 North website. But, he added, "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."
Crowds dressed in thick winter coats gathered outside a large video screen near a Pyongyang train station to cheer and take video and photos on their mobile phones as the state TV anchor, dressed in a pink traditional Korean hanbok, delivered the statement.
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. After several failures, it put its first satellite into space with a long-range rocket launched in December 2012.
Experts say that ballistic missiles and rockets in satellite launches share similar bodies, engines and other technology. The U.N. called the 2012 launch a banned test of ballistic missile technology.
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. But there is a growing debate on just how far the North has advanced in its secretive nuclear and missile programs.
In the first indication of a possible test, the U.S. Geological Survey measured an earthquake Wednesday morning with a magnitude of 5.1. An official from the Korea Metrological Administration, South Korea's weather agency, said the agency believed the earthquake was caused artificially, based on an analysis of the seismic waves and because 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 test is a surprise, both in its purported type and its timing.
North Korea hadn't conducted an atomic 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 strong U.N. sanctions.
The size of Wednesday's quake is bigger than seismic activity reported in previous tests. South Korea's Yonhap news agency reported that quake monitoring agencies detected magnitudes of seismic activity of 3.7 in 2006; 4.5 in 2009 and 4.9 in 2013.
After the North's third atomic test, in February 2013, Pyongyang launched a campaign of bellicose rhetoric that included threats to launch a nuclear attack on the United States and Seoul. North Korea claimed in 2013 that it had scrapped the 1953 armistice that ended fighting in the Korean War. Pyongyang has also restarted a plutonium nuclear reactor shuttered after a 2007 nuclear deal that later fell apart.
Pyongyang says its nuclear weapons program is necessary to defend itself against the United States. North Korea under leader Kim Jong Un has pledged to bolster its nuclear arsenal unless Washington scraps what Pyongyang calls a hostile policy.
Washington sees North Korea's pursuit of nuclear weapons and ballistic missiles as a threat to world security and to its Asian allies, Japan and South Korea.
North Korea, analysts say, needs nuclear tests for practical military reasons. Such tests advance its aims to build nuclear-tipped missiles that can be used as deterrents 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.
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.
The test comes amid failed diplomatic efforts to persuade the North to give up its nuclear ambitions. 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.