After the state earthquake monitoring station detected a seismic tremor estimated at 4.9 to 5.0 on the Richter scale around noon near Punggye-ri in the communist country's northeastern tip -- the location of previous atomic tests -- South Korea immediately stepped up its military preparedness and surveillance status in case of emergency situations and provocations.
"We are closely watching the current situation because the North could conduct additional nuclear tests or fire off missiles," ministry spokesman Kim Min-seok said in a briefing. "South Korea and the United States are mobilizing all available intelligence sources to monitor (North Korea)."
Analysis is currently underway to figure out the size of the nuclear blast, which the ministry estimated to have created 6-7 kilotons of yield. Considering the seismic tremor was 4.9, the blast was deemed "destructive" by the ministry.
One kiloton is equivalent to 1,000 kilograms of TNT explosives. North Korea's first nuclear test involved a weapon that had an estimated yield of about 1 kiloton, while its second may have been a 2 to 6 kiloton device.
Earthquake monitoring stations detected seismic tremors accompanied by low-frequency sound waves that are different from those created by natural earthquakes, officials said.
The earthquake was detected hours after the North's state media earlier in the day said that members of the ruling Workers' Party Political Bureau are working on future rocket launches as part of an "all-out action of high intensity." The report did not make any mention of a nuclear test.
Defense Minister Kim Kwan-jin will meet with U.S. Forces Korea Commander Gen. James Thurman and American ambassador Sung Kim at around 3 p.m. at his office to discuss measures to deal with the North's nuclear test, according to officials.
While North Korea exploded plutonium bombs in 2006 and 2009, it is not yet known whether Pyongyang tested a uranium-based bomb this time or a combination of uranium and plutonium devices simultaneously detonated in separate tunnels.
It could take days to collect air samples from the test site to determine its fission materials, the spokesman said.
If progress has been made after the isolated state revealed its uranium enrichment centrifuge plant to an American nuclear expert in November 2010, Seoul officials believe the North will be capable of testing a bomb out of uranium, which could open a second route to conduct an atomic test and make weapons.