North Korea is believed to have conducted its third nuclear test in defiance of repeated international warnings, after South Korea's authorities detected an unusually powerful seismic tremor near the North's nuclear site in its northeast shortly before noon Tuesday.

Seoul's Korea Meteorological Administration (KMA) said it detected a magnitude 5.0 tremor at 11:57:50 a.m. with its epicenter located in Kilju County, North Hamgyeong Province, which is home to the North's Punggye-ri nuclear test complex.

A senior official at Seoul's foreign ministry said the government saw a "high possibility" that North Korea may have conducted an atomic test, as meteorological authorities in the United States, China, Japan and Russia issued reports of an artificial earthquake in North Korea.

South Korean President Lee Myung-bak convened an emergency meeting of the National Security Council, an official said, as the artificial tremor in North Korea suggested that the communist nation went ahead with its threatened nuclear test.

Government sources said that the size of the tremor detected is stronger than the magnitude 4.5 quake picked up by Seoul after the communist country detonated its second nuclear device on May 25, 2009.

The KMA said it is impossible to determine precisely if the tremor was caused by a nuclear detonation at this juncture.

"To be more precise, there is a need to check data on the air shock waves and see if radionuclides were released into the atmosphere," an official said.

The North has repeatedly threatened to conduct a test since late last month in response to the U.N. Security Council condemned the country's launch of a long-range rocket on Dec. 12.

South Korea' military, meanwhile, has been placed on high alert after the tremor was detected with sources saying they are working closely with the U.S. forces to gather critical intelligence. The alert level for South Korea's military has been raised from 3 to 2, with 1 being the highest. (Yonhap News)