Seoul: South Korea's military said North Korea 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 South Korean Defence 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 analysing details on what type of nuclear material was used and whether it was successful. The official said the 5-magnitude tremor can put the yield of this test at about 10 kilotons, which is believed to be Pyongyang's most powerful nuclear detonation to date. The estimated yield of the fourth nuclear test was six kilotons. South Korea's Joint Chiefs of Staff (JCS) said an artificial quake of magnitude 5 was detected around 9.30 a.m. at North Korea's main Punggye-ri nuclear test site. The European Mediterranean Seismological Centre upgraded the seismic tremor's magnitude from an initial 5 to 5.3. The US Geological Survey said the tremor was a 5.3-magnitude earthquake. The US monitoring agency said the magnitude was caused by an explosion but it could not determine what type of explosion it was. A 5-magnitude artificial tremor was recorded before Pyongyang's fourth nuclear test on January 6 at Punggye-ri where all of the four North Korea underground nuclear tests had been carried out since 2006. A government source was quoted by Yonhap as saying that North Korea may have conducted another nuclear test to mark the country's National Day. Pyongyang had been preparing for its fifth nuclear test ahead of the 68th birthday of the country on September 9.