SEOUL --?The Republic of?Korea (ROK)'s military believed the Democratic People's Republic of Korea (DPRK) may have conducted the "most powerful" nuclear test so far on Friday after an artificial earthquake was detected at a site where its fourth nuclear test was carried out earlier this year.

A?ROK Defense Ministry official was quoted by Yonhap news agency as saying that it was a nuclear test according to a preliminary analysis, but the military is analyzing details on what type of nuclear material was used and whether it was successful.

The official said the 5.0-magnitude seismic tremor can put the yield of this test at about 10 kilotons, which is believed to be the DPRK's most powerful nuclear detonation to date. The estimated yield of the fourth nuclear test was 6 kilotons.

ROK's Joint Chiefs of Staff (JCS) said earlier that an artificial quake of magnitude 5.0 was detected at about 9:30 a.m. local time (0030 GMT) at the DPRK's main Punggye-ri nuclear test site.

The European Mediterranean Seismological Center upgraded the seismic tremor's magnitude from an initial 5.0 to 5.3. The US Geological Survey saw the tremor as a 5.3-magnitude earthquake.

The US monitoring agency said the magnitude was caused by an explosion, but it said it could not determine what type of explosion it may be.

A 5.0-magnitude artificial tremor was recorded before Pyongyang's fourth nuclear test on Jan. 6 at Punggye-ri where all of the four DPRK underground nuclear tests had been carried out since 2006.

An unnamed government source was quoted as saying that the DPRK may have conducted another nuclear test to mark the country's National Day.

According to media reports, Pyongyang had been preparing for its fifth nuclear test ahead of the 68th birthday of the country on Sept. 9.

It would mark the fifth nuclear detonation by the DPRK following the first in October 2006, the second in May 2009, the third in February 2013 and the fourth in January this year.

Prime Minister Hwang Kyo-ahn called an emergency meeting of the National Security Council (NSC) following the suspected test, the presidential office Cheong Wa Dae said. President Park Geun-hye is on her overseas trip to Laos.

Pyongyang has conducted a series of ballistic missile launches since top DPRK leader Kim Jong Un gave an order on March 15 to test a nuclear warhead and ballistic rockets capable of carrying the warhead "in a short time."