South Korea's weather agency says that a three-point-two magnitude earthquake hit North Korea near its nuclear test site and it was presumed to be a natural quake.

The Korea Meteorological Administration said on Sunday that the tremor occurred at 5:29 p.m. Saturday about six kilometers north northwest of the country's Punggye-ri nuclear test site where the North conducted its sixth nuclear test September third.

The agency upgraded the scale of the earthquake from its earlier assessment of three-point-zero magnitude, adding that the tremor originated from a depth of less than two kilometers.

An agency official said that the quake was presumed to have occurred naturally, noting that a sound wave, which is usually generated in the event of an artificial earthquake, was not detected.

The China Earthquake Networks Center also concluded that the quake was not a nuclear explosion and had the characteristics of a natural tremor.

China’s official Xinhua News Agency reported that the center reached the conclusion after an analysis of the sound waves caused by the quakes detected following the North’s nuclear tests in the past.

The center had said earlier that the magnitude three-point-four quake was due to a “suspected explosion,” noting that the tremor originated from a depth of zero kilometers.

Meanwhile, the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) said that the shallow tremor was likely an aftershock from the North’s sixth nuclear test.

CTBTO Executive Secretary Lassina Zerbo told AFP on Saturday that the most probable hypothesis at present is that this is a consequence of the previous event, which was of a significant magnitude and may still have repercussions in a fracture zone.