The Japanese government said it has confirmed that the earthquakes detected were a nuclear explosion and has registered a protest with the North Korean embassy in Beijing, according to Foreign Minister Taro Kono.
North Korea did not comment immediately but Pyongyang state television said it would carry an important announcement at 06:30 GMT.
The website of the US Geological Survey said a first tremor of 6.3 magnitude had a depth of 23 kilometres, an alert that came after Pyongyang said it has developed a more advanced nuclear weapon - a hydrogen bomb that could be loaded into the country's new intercontinental ballistic missiles, that has "great destructive power".
"Possible explosion, located near the site where North Korea has detonated nuclear explosions in the past," said the USGS. "If this event was an explosion, the USGS National Earthquake Information Center cannot determine its type, whether nuclear or any other possible type."
China said it had also detected a second quake in North Korea of magnitude 4.6.
A statement on the China's Earthquake Administration said the second tremor, which it termed a "collapse", came eight minutes after the first quake. The coordinates of the two quakes were almost identical, according to figures provided by the administration.
Previous recent tremors in North Korea have been caused by nuclear tests.
|South Korean newsreaders of the Yonhap agency report on the latest developments [Reuters]|
Of the first shake, the South Korean military confirmed that an artificial quake took place near North Korea's nuclear test site and said it has put its nuclear crisis response team into operation, according to the Yonhap news agency.
South Korean experts reportedly said Sunday's apparent test was more powerful than the 10-kiloton test carried out a year ago.
Richard Broinowski, former Australian ambassador to Seoul, told Al Jazeera that North Korea was sending a stern message to the United States that it has a deterrent and Washington should be careful about making threats.
"There has been a lot of exaggerating recently about the capabilities of the missiles the North possesses and has been testing," he said.
"The Bulletin of the Atomic Scientists suggested that the Hwasong-12 and the Hwasong-14 weapons don't yet have the capacity to reach the US because none are equipped with the weight that a nuclear weapon would need at the tip of them, they're just empty cartridges.
"But it still doesn’t change the fact that in the future [North Korea will] have a thermonuclear weapon and a nose cone that can keep it safe from the stress of re-entry into the atmosphere. They're very close, probably only a few months away, they constantly surprise us."
Japanese Prime Minister Shinzo Abe was quick to respond, saying a confirmed nuclear test would be "absolutely unacceptable".
"We have to strongly protest it," Abe said.
|Japan's meteorological agency officer Toshiyuki Matsumori speaks during a press conference at the agency's headquarters in Tokyo [Kazuhiro Nogi/AFP]|
Al Jazeera's Scott Heidler, reporting from Tokyo, said Japanese officials were likely to be especially nervous.
"This comes just days after a ballistic missile flew over Japanese territory," he said.
"In the days following that missile test, there were several phone calls between Abe and US President Donald Trump, the last one which was just earlier Sunday morning.
"Japanese media are reporting that in that phone call, both leaders agreed that more pressure needs to be put on North Korea - and that was before the news of this potential nuclear test."
Japan's ministry of defence recently requested "its biggest budget in history," said Heidler.
"The ministry says part of that budget will be spent to get a more sophisticated land-based altitude missile system, so they're focused on a bigger and more important defence in the face of North Korea, and this latest incident will only make to want to them pursue that further."
According to the Reuters news agency, witnesses in the Chinese city of Yanji, on the border with North Korea, said they felt a tremor that lasted roughly 10 seconds, followed by an aftershock.
The United States has repeatedly urged China, North Korea's sole major ally, to do more to rein in its neighbour.
North Korea last year conducted its fourth and fifth nuclear tests, saying the fourth in January 2016 was a successful hydrogen bomb test, although outside experts say the claim has not been proved.
Impoverished North Korea and the rich, democratic South Korea are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty.
North Korea regularly threatens to destroy South Korea and its main ally, the United States.
There were slight variations reported of the first earthquake's measures.
Like the USGS, China's Earthquake Administration said it detected a 6.3 magnitude earthquake but reported a depth of zero kilometres. South Korea's Joint Chiefs of Staff, meanwhile, reported a magnitude 5.6 quake.
The Chinese administration said that the first quake was recorded around 11:30am (03:30 GMT).
Al Jazeera's Wayne Hay, reporting from Seoul, said: "The size of this [first] earthquake does vary upon which organisation is reporting it.
"The location of the earthquake was in the northeast of the country and there had been reports in the past few weeks, citing South Korean intelligence, there was movement at that site. So they were suspecting that maybe [North Korea was] preparing for another nuclear test.
"There was a lot of expectation they would conduct their nuclear test on September 9, the date they did it last year, to mark Foundation Day, the founding day of [North Korea]."
Se-Woong Koo, a reporter at the Seoul-based magazine Korea Expose, told Al Jazeera that a sixth nuclear test has been expected.
"The president of South Korea [Moon Jae-In] convened a meeting of the National Security Council some 30 days ago," he said.
"China and Russia have both been clear that while they're displeased and alarmed by the North's weapons' development, they also think it's foolish for the US and South Korea to pursue more military exercises.
"They want all parties to calm down, but it's clear the North is not interested in that, and it's not entirely [certain] if the US is interested in calming the situation either."
Source: Al Jazeera and news agencies