A 6.3 magnitude earthquake in North Korea on Sunday appeared related to a nuclear test, South Korea’s Yonhap News reported, citing the presidential office.
The report comes after an earthquake in the northeast region of North Korea.
A nuclear test would be a provocation that follows successive launches of intercontinental ballistic missiles capable of reaching the U.S. It would be the sixth nuclear test by Pyongyang since 2006 and the first since the U.S. and South Korea elected new leaders.
Tests of ICBMs in July brought Kim Jong Un’s isolated regime a step closer to achieving its aim of being able to deploy a nuclear warhead over the continental U.S. Last Tuesday, Pyongyang fired a missile over northern Japan into the Pacific Ocean.
The news comes just hours after North Korea said it had developed a more advanced nuclear weapon that has "great destructive power" and that dictator Kim Jong Un had inspected a hydrogen bomb that would be loaded on a new intercontinental ballistic missile.
The state-run KCNA news agency said that the hydrogen bomb's power is adjustable to hundreds of kilotons and can be detonated at high altitudes, with indigenously produced components that would allow the country to build as many nuclear weapons as it wants.
The disclosure comes after Pyongyang fired a missile that flew over Japan and landed in waters off the northern region of Hokkaido early Tuesday — marking a sharp escalation of tensions on the Korean peninsula.
That test of the intermediate-range Hwasong 12 missile came as U.S. and South Korean forces conduct annual military drills on the peninsula, which North Korea had strongly opposed.
North Korea also threatened last month to fire missiles into the sea near Guam after President Donald Trump warned Pyongyang would face "fire and fury" from a "locked and loaded" military if it threatened the United States or any ally.
Retired Air Force Lt. Col. Rick Francona said Saturday that "if it's true, this is a major step forward.
"We've been watching this for some time now," he told Ana Cabrera on CNN. "We know they developed an intercontinental ballistic missile because we can see that.
"What we cannot see is actually the development of a warhead.
"If they have been able to miniaturize a nuclear weapon down to the size that they can put it on the top of an ICBM, they've got a real capability to strike the United States."
Francona, 66, who also served the NSA and the CIA, said that Pyongyang's news report would trigger strong "intelligence collection" for verification but that "then the problem becomes, what do you do about it?
"That opens up a whole other series of questions that we've been talking about the last few weeks."
In addition, Francona characterized the unverified report as "just the tit-for-tat, back and forth between the United States and North Korea.
"We do something. They do something in return. Then, we do something. It just gets ratchets up the temperature—and, of course, we don't need this.
"What we need to do is figure out some way to deal with this situation rather than just threatening them."
Bloomberg News contributed to this report.
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