President Lee Myung-bak convened an emergency security session. His spokesman, Lee Dong-kwan, confirmed that a nuclear test may have been carried out in the North.
Seismologists from the U.S., South Korea and Japan reported earthquakes in an northeastern area, where North Korea conducted a nuclear test in 2006.
A 4.7-magnitude earthquake was registered in northeastern North Korea at 9:54 a.m. (0054 GMT), the U.S. Geological Survey said.
The quake, measured at a depth of 6 miles (10 kilometers) underground, occurred 40 miles (70 kilometers) northwest of the city of Kimchaek, the USGS said.
The Japan Meteorological Agency also said it detected seismic activity Monday morning. "We are checking whether they were due to a nuclear test," agency official Gen Aoki said in Tokyo.
In Seoul, the Korea Institue of Geoscience and Mineral Resources reported a 4.5-magnitude quake in Kilju in North Hamgyong Province.
North Korea also carried out a nuclear test in October 2006 in Kilju, a test that drew sanctions from the United Nations and prompted five other nations to push negotiations on a nuclear disablement-for-aid pact with North Korea.
In Washington, State Department spokesman Andy Laine said the U.S. government had no confirmation of a new nuclear test.
"At this point we've seen the reports and we're trying to get more information, but we're not able to confirm at this time," Laine said.
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