The US Geological Survey said it had detected a magnitude 4.7 tremor in North Korea at 0954 local time on Monday, indicating that a nuclear test may have taken place.
It located the epicentre of the tremor about 375km northeast of the North Korean capital, Pyongyang.
|The test site is believed to be near the northeastern town of Kilchu
South Korean media said the country's president, Lee Myung-bak, had called an emergency cabinet meeting on Monday morning to discuss the South's response.
The reports come amid escalating tensions on the Korean peninsula and follows North Korea's controversial rocket launch in April.
The launch triggered a tightening of international sanctions against North Korea which in turn provoked an angry reaction from the North, which said it was pulling out of nuclear disarmament talks and restarting its weapons programme.
It has also repeatedly threatened to conduct a new nuclear test.
North Korea conducted its first nuclear test in October 2006, although it was unclear whether that device successfully detonated.
North Korea is believed to have extracted enough weapons grade plutonium for about eight bombs.
Donald Kirk, a Korea expert based in the South Korean capital, Seoul, said the test was almost certain to raise tensions in the region and "raises the spectre of a nuclear arms race in northeast Asia".
He said that the test could raise pressure on the governments of Japan and Taiwan, among others, who may be tempted to develop their own nuclear weapons.