In an undated image distributed on Sept. 3, 2017, North Korean leader Kim Jong Un was inspecting the loadin

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.