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 later said the tremors were a nuclear explosion, Reuters reported.

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.

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.

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.

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.

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.