(CNN) -- North Korea has conducted a sixth nuclear test, the Japanese government said, a move the United States and its allies in the region are likely to view as a major provocation.
Seismological data from the United States Geological Survey (USGS) showed that an explosion caused a 6.3-magnitude tremor in the country's northeast, not far from the country's Punggye-ri nuclear test site.
If the initial data holds, it would make it the most powerful weapon North Korea has ever tested. The last nuclear test Pyongyang conducted, which was nearly a year ago, triggered a 5.3-magnitude seismological event.
"After analyzing data provided by the Japan Meteorological Agency, the Japanese government concluded that North Korea has conducted a nuclear test," Japanese Foreign Minister Taro Kono said in a live television broadcast.
North Korean state media said it will make a major announcement at 3 p.m. Pyongyang time (2:30 a.m. Eastern Time).
South Korea and Japan are gathering and analyzing data to confirm details of the test, which Japanese Minister Shinzo Abe said could not be tolerated.
"If North Korea did indeed conduct a nuclear test, we absolutely cannot tolerate and must protest firmly. We will convene a National Security council meeting to gather and analyze the information," Abe said in a live television broadcast prior to Kono's announcement.
South Korea is holding a National Security Council meeting to discuss the incident, presided by President Moon Jae-in, according to South Korea's presidential office. Japan has dispatched so-called "radiation sniffer" planes to take samples to confirm that a nuclear explosion did in fact take place.
South Korea's Joint Chiefs of Staff said in a statement that the military had "detected a man-made earthquake near Punggye-ri and is analyzing whether it was a nuclear test."
It also announced that the military has raised its alert status.
Japan Meteorological Agency also observed a magnitude-6.1 tremor in North Korea, which showed a different waveform from a natural quake around 12:31 p.m. local (11:31 p.m. ET).
A second, smaller tremor was detected by the USGS and its Chinese counterpart. Both speculated it may have been caused by some sort of structural collapse.
North Korea's weapons program has been progressing at a rapid pace under leader Kim Jong Un.
The country tested two nuclear weapons last year, including one in September close to the country's Foundation Day holiday.
This would be the first nuclear test under the administration of US President Donald Trump, which says it is pursuing what it calls a strategy of "peaceful pressure" to get North Korea to bring its nuclear weapons program to the negotiating table.
But critics worry that the Trump administration's insistence on denculearization and some of the President's harsh rhetoric have done more harm than good.
"The test is dramatic, but not necessarily a game-changer. Thermonuclear weapons are more destructive, but their use can be deterred with the same actions as smaller warheads. The critical thing is now is that the United States does not cause more damage with its reaction than the test did itself," said Adam Mount, a North Korea expert and senior fellow at the Center for American Progress.
North Korea sees its nuclear weapons program as crucial to deterring any US-led attempts at invasion, making it unlikely that they will negotiate it away, analysts say.
"It's too late to think we could create enough pressure and pain that they will volunteer to eliminate their arsenal. The priority now is to maintain stability, to lower the risk of miscalculation and miscommunication so that a war doesn't break out that nobody wants," Mount said. "Denuclearization is a long-term goal, not a short-term solution."
Tensions between North Korea and the international community flared against last week after Pyongyang flew a missile over Japan. The United States and its allies responded by sending fighter jets and bombers over the Korean Peninsula in a so-called "show of force" operation.
The test comes just hours after the country released images of Kim inspecting what it said was a hydrogen bomb ready to be fitted atop a missile with intercontinental capabilities.
Pyongyang test-fired two missiles last month that experts say could potentially deliver a nuclear warhead to the United States.
"We shouldn't be surprised by the fact they had the test, but usually they spread it out a bit," says Melissa Hanham, a senior research associate in the East Asia Nonproliferation Program (EANP) at the James Martin Center for Nonproliferation Studies.
If the current magnitude holds, it's possible that the weapon tested Sunday was a thermonuclear device, but it's impossible to say if it was the peanut-shaped object shown in the images released by North Korean state media, says Hanham.
"When you're evaluating these explosions, you have to do it with a grain of salt because you're trying to mold a mathematical equation to something that's happening in the real world. And the equations can to grip on things like depth or the geology of the area, and we don't know those kinds of facts," she said.
Experts have said for months that another nuclear test was likely on the way, with satellite imagery revealing that a new tunnel had been dug earlier this year.
Independent nuclear experts told CNN that another test would likely be a way to increase the yield, or destructive power, of a weapon.
North Korea claimed that it set off a thermonuclear weapon during the September 2016 test, but experts said the data showed it was more likely boosted fission weapon. Thermonuclear weapons typically use a fission explosion to create a fusion reaction, which is far more powerful than a fission reaction.
"For months North Korea refrained from conducting a nuclear test and from launching missiles over Japan, even though that meant testing on highly lofted trajectories and launching satellites to the south rather than east, which is more efficient. It now seems to have decided to end that restrain," said David Wright, the co-director of the Union of Concerned Scientists Global Security Program.
"While this is likely to make it more difficult for the US and its allies to pursue diplomacy, that remains the best of the options on the table for trying to reduce the level of tension and avoid a crisis."
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