North Korea appears to have conducted a fifth nuclear test Friday, authorities in Japan and South Korea said, after monitors detected a 5.3-magnitude "artificial earthquake" near its main nuclear site.
A confirmed test by the isolated North would send tensions soaring over its nuclear and ballistic missile ambitions, which have already earned international condemnation and United Nations sanctions.
The quake was detected near North Korea's Punggye-ri nuclear test site as the country celebrates Foundation Day, which marks the anniversary of the founding of the nation in 1948.
Authorities in Japan and South Korea said the tremor, measured at 5.3 by the US Geological Survey, had the hallmarks a nuclear test carried out on January 6 at Punggye-ri.
"The artificial... earthquake in the North... is likely a nuclear test," a South Korean meteorological agency official said, according to the Yonhap news agency.
"There is a high possibility that it was a nuclear test, given the location and the magnitude of the quake," another unidentified government official told Yonhap.
Japan's Chief Cabinet Secretary Yoshihide Suga told an emergency press conference that the government was acting swiftly to gather information.
"We think that there is a possibility that this quake occurred because North Korea carried out a nuclear test," he said.
Public broadcaster NHK reported Japan's defence ministry was preparing to dispatch aircraft to analyse air samples to see if any radiation could be detected.
Foreign Minister Fumio Kishida vowed that if a nuclear test is confirmed, Japan will "lodge a strong protest against North Korea... and immediately respond at the UN Security Council."
North Korea has been hit by five sets of United Nations sanctions since it first tested a nuclear device in 2006. Since the January test, concern has also grown over a series of provocative ballistic missile launches.
Pyongyang test-fired three missiles Monday as world powers gathered for a G20 meeting in China, with leader Kim Jong-Un hailing the tests as "perfect" and US President Barack Obama warning it would only up the pressure.
North Korean nuclear tests are usually heralded by chatter among analysts about preparations at Punggye-ri but there had been little discussion in recent weeks over any signs of a test.
"The test caught many off guard, although Seoul officials have for months maintained that the North was maintaining a condition in which it was ready to stage a nuclear test any time," said Kim Jin-Moo, an analyst at the Korea Institute for Defense Analyses.
The US-Korea Institute at Johns Hopkins University said last month on its closely watched website, 38 North, that there had been "continued activity" at the site, particularly at the location where the January test took place.
A test would be another slap in the face to the North's chief ally China and diminishes any chance of a resumption of six-country talks on North Korea's nuclear programme.
North Korea claimed its January test was of a miniaturised hydrogen bomb, which is far more powerful than other nuclear devices, although experts said seismic activity suggested it was not strong enough.
Scientists say the estimated yield of around six kilotons was similar to the North's last nuclear test in 2013 and far too low for a powerful thermonuclear device.
But at a rare party congress in Pyongyang in May, Kim Jong-Un hailed what he called a "historic" landmark, and insisted it had been a hydrogen blast.
He praised the country's scientists for "creating milestone miracles with the magnificent and exhilarating sound of the first H-bomb of our republic".
The test "clearly demonstrated to the whole world our undefeatable spirit and endless power ... in defiance of malicious pressure and sanctions by enemy forces," he said.