Japan has concluded that the tremors detected in North Korea were a nuclear explosion, marking the sixth atomic test by Pyongyang since 2006.
The shallow, 6.3 magnitude earthquake shook North Korea today, suggesting it had detonated a sixth nuclear device, hours after it said it had developed an advanced hydrogen bomb that possesses "great destructive power".
"After examining the data we concluded that it was a nuclear tests," Foreign Minister Taro Kono said at a briefing broadcast by public broadcaster NHK following a meeting of Japan's National Security Council.
North Korea's sixth nuclear test is the first since President Trump was inaugurated.
The action probably will increase already high tensions between the Trump administration and Kim Jong Un's regime.
The US Geological Survey said it had recorded a 6.3 magnitude earthquake at noon Sunday local time, near North Korea's known nuclear test site in the country's northeast region.
The quake was felt in northern China, with emergency sirens blaring in Yanji, near the North Korean border, according to local media.
China's Earthquake Administration said it detected a second quake in North Korea of magnitude 4.6, which it termed as a "collapse".
The administration's website said the second quake, measured at a depth of zero kilometres, came eight minutes after the first quake, which it said was a "suspected explosion".
The co-ordinates of the two quakes were almost identical, according to figures provided by the administration.
The first quake struck 75km north northwest of Kimchaek. Previous recent tremors in the region have been caused by nuclear tests, which if the case this time round, is bound to increase the tension hours after US President Donald Trump and Japanese Prime Minister Shinzo Abe talked by phone about the "escalating" nuclear crisis.
North Korea's recent nuclear tests have also happened exactly on the hour, often on meaningful dates for North Korea or the United States. It is a holiday weekend in the United States, which marks Labor Day on Monday.
A North Korean nuclear test in September of last year registered as a 5.3-magnitude earthquake at 9am. on a national holiday marking the 68th anniversary of the formation of the communist regime by Kim Il Sung, the current leader's grandfather.
South Korean authorities said it appeared to be artificial, consistent with a nuclear test.
President Moon Jae-in immediately called an emergency meeting of his national security council.
In Japan, Shinzo Abe said he "would not tolerate" another nuclear test.
All the components of the "H-bomb" were "homemade" so North Korea could produce "powerful nuclear weapons as many as it wants," the state-run Korean Central News Agency quoted Kim as saying.
North Korea's latest claim on a hydrogen bomb could not be verified. Although it claimed that a nuclear test in January last year was of a hydrogen bomb, experts said the seismic waves generated were consistent with an ordinary nuclear device, not a thermonuclear one.
A sixth nuclear test by the North Koreans would be highly inflammatory for the United States and China, as well as North Korea's other neighbors.
China has expressed annoyance at North Korea's frequent ballistic missile launches, but analysts have said China probably would not take serious action - unless there is another nuclear test.
Today's events could also change the equation in Washington. Trump has been warning the Kim regime not to test him, warning on Twitter that the American military was "locked and loaded."
- With AP