A South Korean Defense Ministry spokesman says Seoul believes that a nuclear test caused an earthquake in North Korea near the site of two previous atomic tests.
Kim Min-seok says North Korea informed China and the United States of its plans to conduct a nuclear test.
Kim says that shortly before noon Tuesday an earthquake was detected in North Korea.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
An earthquake was detected Tuesday in North Korea just north of a site where the country has conducted nuclear tests, providing strong indication that Pyongyang has gone ahead with a highly anticipated third test. There was, however, no confirmation of the test.
The South Korean Defense Ministry, which raised its military alert level after the quake, said it was trying to determine whether it was a test. Nuclear blasts can create tremors but they are distinct from those caused by natural earthquakes.
The U.N. organization monitoring nuclear tests says it has detected an "unusual seismic event" in North Korea.
The U.S. Geological Survey as well as earthquake monitoring stations in South Korea detected an earthquake just north of a site where North Korea conducted its second nuclear test in 2009.
"There is a high possibility that North Korea has conducted a nuclear test," said Chi Heoncheol, an earthquake specialist at the government-funded Korea Institute of Geoscience and Mineral Resources. Chi said a magnitude 3.9 magnitude earthquake and a magnitude 4.5 earthquake were detected in the North's 2006 and 2009 nuclear tests.
South Korean, U.S. and Japanese seismic monitoring agencies put the magnitude of Tuesday's quake at 4.7, 4.9 and 5.2 respectively.
"We think it is possible it came about as a result of a nuclear test by North Korea from looking at past cases," Japanese government spokesman Yoshihide Suga said at a news conference Tuesday, noting that they were still gathering information.
The United States and its allies have been on edge since North Korea said last month it will conduct its third nuclear test to protest toughened sanctions over a December rocket launch that the U.N. called a cover for a banned missile test.
North Korea's politburo vowed to continue firing "powerful long-range rockets," but a statement by state media Tuesday made no mention of a nuclear test.
North Korea's National Defense Commission said Jan. 23 that the United States was its prime target for a nuclear test and long-range rocket launches. North Korea accuses Washington of leading the push to punish Pyongyang for its December rocket launch.
Last October, a spokesman from the commission told state media that the country had built a missile capable of striking the United States, but did not provide further details. A missile featured in an April 2012 military parade appeared to be an intercontinental ballistic missile, but its authenticity has not been verified by foreign experts.