North Korea made what it called "important announcement" through its state-run TV at 3 p.m. that it has successfully conducted the test of hydrogen bomb warhead designed for intercontinental ballistic missiles, though South Korean authorities are yet to confirm Pyongyang's allegation.

According to the Korean authorities and the media, a tremor with the 5.7 magnitude was detected in Gilju-gun, North Hamgyeong Province, at around 12:20 p.m. on Sunday. Another quake on scale of 4.6 was reportedly detected eight minutes later on the test site.

The United States Geological Survey (USGS) said the first tremor was 6.3 on the exponential scale, suggesting a far bigger test than before. The location of the epicenter was near the site where the North had detonated nuclear explosions in the past.

President Moon Jae-in convened a National Security Council session, shortly after the "artificial earthquake" from North Korea's nuclear test site was detected. 

Chinese authorities also confirmed the detection of the first quake, measuring its magnitude at 6.3. China's related agency also said it detected the North's second quake of 4.6 magnitude.

However, the Korean Meterological Administration said it did not detect any sign of the second quake, contradicting China's announcement. The KMA initially announced the first quake was 5.6 magnitude. It adjusted the magnitude to 5.7. 

The weather agency said the North's artificial quake on Sunday is about 5 to 6 times more powerful than the fifth nuclear test last year.

South Korean military has raised alert level following the reports of North Korea's apparent nuke test.

"North Korea appears to have carried out a nuclear test," Japanese Foreign Minister Taro Kono told Japanese reporters Sunday afternoon after participating in a top-level security meeting. 

Japanese Prime Minister Shinzo Abe said he "cannot tolerate the North's move (if the quakes turn out to be a nuclear test), expressing his strong protest against Pyongyang.

The latest quakes in North Korea are raising fears that the North is accelerating its nuclear test amid the growing tension on the Korean Peninsula. North Korea conducted its first nuclear test in 2006.

The tremor's detection came hours after North Korean leader Kim Jong-un inspected the loading of a hydrogen bomb into a new intercontinental ballistic missile.

(khnews@heraldcorp.com)