North Korea was struck by as many as two strong tremors on Sunday, with the South's Joint Chiefs of Staff saying it was presumed to be a sixth nuclear test.
"By way of the embassy in Beijing, we made it clear in the strongest words, that if this is a nuclear test, this is unforgivable, and a violation of the U.N. Security Council Resolution," Kono said, according to a CNBC translation
The report cited the USGS as saying the first tremor occurred at around 12 noon North Korea time.
The defense committee chairman in South Korea's parliament said the blast likely yielded 100 kilotons, which would be around four or five times the size of the bomb dropped on Nagasaki in 1945, Reuters reported, citing the South's official news agency Yonhap.
Meanwhile, Japan's meteorological agency said the tremors were at least ten times as powerful as the North's previous test, which was in September of last year, Reuters reported. But South Korea's meteorological agency put the energy at five to six times the fifth nuclear test, Reuters reported.
Jeffrey Lewis, a director of the East Asia Nonproliferation Program at Middlebury Institute of International Studies at Monterey, said on Twitter on Sunday that the size of the earthquake indicated that the explosion could be estimated at ranging from around a few hundred kilotons to around a megaton.
China's earthquake administration later said it detected a second quake of magnitude 4.6 at a depth of zero kilometers, which it called a "collapse," Reuters reported, noting the second tremor came eight minutes after the first at nearly identical coordinates.
But the South Korean Meteorological Administration said it didn't detect a second quake, Dow Jones reported, although Reuters later reported that witnesses in a Chinese city near the border with North Korea felt a tremor followed by an aftershock.
USGS said it detected a second seismic event of magnitude 4.1, which it said was likely a "secondary feature," and possibly a structural collapse.
A South Korean military official told NBC News that the first reported quake was artificial, while Yonhap reported the military said it was located near the North's nuclear test site.
South Korea's Blue House, the country's equivalent of the White House, said that the North may conducted another nuclear test, its sixth, Yonhap reported.
The South's President Moon Jae-in has called a National Security Council meeting, while the country's military raised its alert level, with all troops on high alert, Yonhap reported.
Previous tremors in the reclusive country have been caused by nuclear tests. The wave form signal for an explosion and an earthquake are different, allowing geologists to distinguish a man-made tremor.
The potential nuclear test followed North Korea saying on Sunday that it had developed a more advanced thermonuclear weapon of "great destructive power," which it would load onto an intercontinental ballistic missile, Reuters reported.
Dr. Koh Yu Hwan, a professor of North Korea studies at Dongguk University in Seoul told NBC News on Sunday that given the North's statement on a hydrogen bomb earlier in the day, the test would likely have involved such a bomb.
That would mean the North had nearly completed its efforts to become a legitimate nuclear power, he said.