North Korea claimed Wednesday that it had tested a “miniaturized” hydrogen bomb less than a month after the country’s dictator boasted that the weapon had been developed.

The surprise announcement further complicates efforts to curb the Stalinist nation’s move toward a working nuclear arsenal and will almost certainly lead to a push for new sanctions at the United Nations.

The announcement was made by a state television anchor who read a typically propaganda-heavy statement calling the test a “perfect success” that elevated the country’s “nuclear might to the next level” and provided it with a weapon to defend against the United States and its other enemies.

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 statement was delivered.

In Washington, State Department spokesman John Kirby said the U.S. could not immediately confirm North Korea’s claim, but warned “we condemn any violation of U.N. Security Council Resolutions and again call on North Korea to abide by its international obligations and commitments.”

North Korea’s neighbors reacted with fury to the announcement. South Korean presidential security official Cho Tae Yong said, “We strongly condemn” the test, adding that the North must abide by U.N. resolutions that require the country to scrap its nuclear and ballistic missile programs completely and irreversibly.

Japanese Prime Minister Shinzo Abe told reporters the test represented a threat to his nation’s safety.

“We absolutely cannot allow this, and condemn it strongly,” said Abe, who added that he would take “strong action” and work with other nations, including the U.S., South Korea, China and Russia, to resolve the situation.

North Korean nuclear tests worry Washington and others because each new blast is seen as pushing North Korea’s scientists and engineers closer to their goal of an arsenal of nuclear-tipped missiles that can reach the United States.

While a hydrogen bomb is much more powerful than an atomic bomb, it is also much harder to make. In a hydrogen bomb, radiation from a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.

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 it believed the earthquake was caused artificially based on their analysis of the seismic waves and that it originated 30 miles north of Kilju, the northeastern area where North Korea’s main nuclear test site is located. 

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. 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 atomic bomb tests. 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.

Since the elevation of young leader Kim Jong Un in 2011, North Korea has ramped up angry rhetoric against the leaders of allies Washington and Seoul and the U.S.-South Korean annual military drills it considers invasion preparation.

Last month, Kim Jong Un said his country was “ready to detonate a self-reliant A-bomb and H-bomb to reliably defend its sovereignty and the dignity of the nation”. The comments were met with skepticism by North Korea watchers, who said it was unlikely that Pyongyang possesed the technology to develop such a weapon.

Fox News Channel’s James Rosen and The Associated Press contributed to this report.