"We've been informed by the South Koreans that there's been a (North Korean) nuclear test," a UN security council diplomat told Reuters on condition of anonymity.
An international nuclear test monitoring agency said the location of the seismic event was "roughly congruent with" 2006 and 2008 tests carried out by the reclusive state and had "clear explosion-like characteristics".
North Korea, which had been threatening a third nuclear test, had informed Beijing and Washington on Monday of plans to undertake a test, South Korea's Yonhap news agency reported.
The isolated state, which is banned under UN Security Council resolutions from developing nuclear and missile technology, did not make any immediate comment.
North Korea is not prone to seismic activity and it may take hours or even days to determine officially whether a nuclear test had been conducted.
South Korea's defence ministry said the North Korean seismic event could be the result of a 6-7 kiloton or stronger nuclear blast. South Korea's President Lee Myung-bak called a national security council meeting for 0400 GMT.
North Korea successfully launched a long-range rocket in December in violation of UN resolutions that banned it from developing missile or nuclear technology after nuclear tests in 2006 and 2009.
It announced plans for a third nuclear test in response to the sanctions imposed in January after the rocket launch, although satellite imagery indicated it has been readying its test site for more than a year.