North Korea Claims It Just Successfully Detonated a Hydrogen Bomb

North Korea has announced that it successfully detonated a small hydrogen bomb, a declaration that followed a suspicious earthquake Wednesday near a site where the country previously conducted three nuclear weapons tests.

A hydrogen bomb, also known as a thermonuclear bomb, uses more advanced technology to produce a significantly more powerful blast than other atomic devices. Successfully testing an H-bomb would be a huge step forward militarily for the North, and a worrying development for Japan, South Korea, and the US, which has several military bases in the region.

Kim Jong-un said in early December that the country had been developing an H-bomb, but the claim was met with widespread skepticism. Wednesday's test occurred two days ahead of the North Korean leader's birthday on January 8, when he will turn 33.

Related: Kim Jong-un Says North Korea Has an H-Bomb — But No One Believes Him

In a broadcast announcing the alleged H-bomb test, North Korean state media said it was "an act of self-defense" against the US and other perceived threats to the country.

"North Korea was forced to develop its nuclear arsenal because of the US's hostile policy against Korea," the North Korean newsreader said, according to a translation provided by the Washington Post. "However, as a peaceful nation and a nuclear powered nation, North Korea will be a responsible state and will not use its nuclear power before [an attack] and will not transfer the technology to others."

Nuclear weapons experts cautioned in the immediate aftermath of North Korea's announcement that the H-bomb test was still unconfirmed.

"North has made a lot of claims about its nuclear program that haven't held up to serious investigation," Melissa Hanham, a senior research associate at the Center for Nonproliferation Studies, told VICE News. She explained that the US was likely in the process of flying as close as possible to the test site to detect radionuclides, which could indicate what type of bomb was used. "At that point they may be able to tell whether it's a quote unquote H-bomb," she said.

We won't know if it's an actual 'H-bomb' until radiation is detected. Even so, we have not always been able to detect it in time.

— Melissa Hanham (@mhanham)January 6, 2016

According to the US Geological Survey (USGS), a 5.1 magnitude earthquake was recorded at around 10am local time in the northeast corner of the country, about 230 miles northeast of Pyongyang. The location is about 30 miles away from from the Punggye-ri site where the North conducted three nuclear tests in the past. The USGS initially said the quake occurred about 10 kilometers below ground, then revised its findings to say it occurred at the Earth's surface.

The last North Korean nuclear test, in 2013, also registered at 5.1 on the USGS scale, and the similar seismic activity caused by the recent blast fueled speculation that North Korea may have detonated a nuclear weapon similar to the ones it tested previously, then tried to pass it off as an H-bomb.

"My belief is that the [seismic] wave from this is similar to previous tests, which were not H-bombs," Hanham said.

North Korean state media described the H-bomb as "miniaturized" (or small, according to some translations), which could account for the lesser seismic waves that were recorded. Hanham noted that the language is significant because making the weapons smaller potentially allows them to be mounted on missiles that could hit targets outside the country.

Related: North Korea Is Slowly Learning How to Launch a Ballistic Missile From a Submarine

"In the past, North Korea has also used the word miniature or miniaturization," Hanham said. "Unfortunately, they know it's kind of a trigger word for the rest of us. It's one thing to have a nuclear weapon, it's another thing to deliver a nuclear weapon. People have said they need to miniaturize it to put it on a missile."

Overlay of seismic waveforms for station MDJ for North Korean tests suggests similar size to 2013 via — Jascha Polet (@CPPGeophysics)January 6, 2016

Dr. Ferenc Dalnoki-Veress, a nuclear physicist who is a scientist in residence at the Middlebury Institute of International Studies at Monterey, said it's possible North Korea has the materials required to make a hydrogen bomb, but unlikely that they have the technology to make it successfully detonate.

"They may have the material, but the question is do the have the know-how, do they have the knowledge of nuclear physics and so on. That I'm not sure of and I personally don't think so," he said.

H-bomb explosions are several orders of magnitude larger than the type of nuclear weapons used by the US on Hiroshima and Nagasaki during World War II. Those devices, which are relatively crude by modern standards, produced explosions that were the equivalent of less than 20 kilotons of TNT. Today's H-bombs are capable of producing blasts greater than 50,000 kilotons.

Dalnoki-Veress said it was possible — though not probable — North Korea's bomb was small because they were simply testing the technology, and not seeking to produce a large explosion. He noted, however, that this would be "very, very hard to do."

Related: No One Really Knows How Many Nukes North Korea Has

"If they say they have a hydrogen bomb It could mean they've produce a very low-yield hydrogen bomb not for the purpose of producing a large explosion, but just to prove they could do it," he said. "It's possible they could do a low-yield test, but that would not be the purpose of a hydrogen bomb, because you want to produce as large an explosion as possible."

South Korea's presidential office convened an emergency security on Wednesday in Seoul. The UN Security Council is also expected to meet on Wednesday to discuss the situation in North Korea.

North Korea's last nuclear test occurred in February 2013 and prompted stiff sanctions from the United Nations aimed at preventing further tests. North Korea's two other nuclear tests occurred in 2006 and 2009.

Experts at 38 North, a site affiliated with John's Hopkins University that tracks North Korea's weapons program, had previously analyzed satellite imagery that it said in December showed the continued excavation of a new nuclear test tunnel at the Punggye-ri site.

The White House said it could not immediately confirm that the test on Wednesday was of a hydrogen bomb.

"While we cannot confirm these claims at this time, we condemn any violation of UNSC Resolutions and again call on North Korea to abide by its international obligations and commitments," White House National Security Council spokesman Ned Price said in a statement.

Reuters contributed to this report

Follow Keegan Hamilton on Twitter: @keegan_hamilton