North Korea says it conducted an underground nuclear test on Monday.

"We have successfully conducted another nuclear test on May 25 as part of the republic's measures to strengthen its nuclear deterrent," the North's official Korean Central News Agency reported.

Lee Dong-kwan, a spokesman for the South Korean president, says that a nuclear test may have been carried out in the North.

President Lee Myung-bak has called an emergency cabinet meeting.

Seismologists from the United States, 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. local time, the U.S. Geological Survey said.

The quake, measured at a depth of 10 kilometres underground, occurred about 70 kilometres 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 Institute of Geoscience and Mineral Resources reported a 4.5-magnitude quake in Kilju in North Hamgyong Province.

In October 2006, North Korea carried out a nuclear test 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.