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.
Japan's Foreign Minister Taro Kono later said in a briefing broadcast by NHK that it concluded the tremors were a nuclear explosion. Defense Minister Itsunori Onodera said during the broadcast that Japan had sent at least three military jets to test for radiation.
"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.
North Korean television later said the reclusive country's leader, Kim Jong Un, had ordered a test of a hydrogen bomb intended to be mounted on an ICBM, Reuters reported.
North Korea called the test a complete success, with a greater yield than previous tests and no adverse impact on the environment, Reuters reported, citing the television report.
A hydrogen bomb is much more powerful than the simpler types of atomic weapons tested by North Korea five times previously, or the bombs dropped on Japan during World War II. H-bombs are also harder to develop.
The USGS initially reported a first tremor as a 5.6 magnitude, but later raised it to 6.3, while China's earthquake administration said it detected a 6.3 magnitude earthquake in Northeastern North Korea, calling it a "suspected explosion," Reuters reported.
The depth of that quake was recorded as zero kilometers, the China earthquake administration said, according to Reuters.
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, Kim Young-woo cited a report from military authorities and said the yield was provisionally estimated at up to 100 kilotons, which would be around four or five times the size of the bomb dropped on Nagasaki in 1945, the South's official news agency Yonhap reported.
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.
Yonhap reported that the year-ago explosion triggered a 5.04 magnitude earthquake with a presumed yield of 10 kilotons.
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 have 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.
South Korea's presidential senior secretary for national defense spoke over the phone with his U.S. counterpart, national security adviser H.R. McMaster, a South Korean presidential spokesperson told NBC.
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.
"We passed the point of trying to resolve North Korean nuclear problem through dialogue," Koh said. "It would be difficult not to give recognition as a nuclear state in order to resolve the nuclear issue now and, certainly, it would be hard to make any progress by asking North Korea to abandon its nuclear ambition at this late stage."
The suspected nuclear test followed Pyongyang's test last week of a missile which flew over Japan in what the U.S. and its allies have called a major provocation.
The U.S. and its allies have sought a diplomatic solution to curb the isolated nation's aggression, enacting both unilateral and multilateral sanctions. In particular, they have sought to apply economic pressure through China, North Korea's only major ally.
Tensions surrounding North Korea's missile tests have ratcheted up throughout the summer as Kim Jong Un and U.S. President Donald Trump engaged in a war of words.
Trump previously warned Pyongyang that threats against the U.S. would be met with "fire and fury." North Korean state media subsequently responded by saying that it was considering striking the U.S. territory of Guam.
—CNBC's Akiko Fujita, Jacob Pramuk and Christine Wang contributed to this article.