The U.S. Geological Survey is reporting a 4.9-magnitude earthquake in North Korea, raising fears that Pyongyang may have gone ahead with its threat to conduct a third nuclear test.
The USGS report put the epicenter of the quake in Kilju county, near where North Korea's Punggye-ri nuclear test site is located.
South Korean officials would not confirm whether the widely anticipated nuclear test had taken place, but several officials says it is the "likely" cause of the quake. North Korea is not prone to seismic activity.
South Korean President Lee Myung-bak has convened a meeting of the National Security Council in response to the suspected test. Japanese Prime Minister Shinzo Abe is also holding an emergency meeting Tuesday.
North Korea has threatened to carry out its third nuclear test in retaliation for United Nations sanctions that were expanded last month in response to a recent long-range rocket launch.
Earlier Tuesday, North Korean state media threatened an unspecified "high-intensity" action, but did not mention any possible nuclear test.
North Korea is banned from conducting nuclear and missile tests under UN sanctions passed in 2006 and 2009.