The USGS initially reported the 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 the first 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.

A South Korean military official told NBC News that the first quake was artificial, while Yonhap reported the military said the first tremor 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, the South’s official news agency 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, 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. (read more)

A 5.6 magnitude earthquake was detected near North Korea's nuclear test site https://t.co/ysmmfgzW3d — New York Post (@nypost) September 3, 2017

Earthquake detected in North Korea feared to be result of nuclear test https://t.co/E7PCMTqSiG via @dcexaminer — Joe FreedomLover🇺🇸 (@JoeFreedomLove) September 3, 2017