There was a "high possibility" that North Korea tested another nuclear weapon Tuesday, South Korea's Yonhap News quoted a Foreign Ministry official as saying, DPA reported.
A magnitude-5.1 earthquake hit North Korea at 11:58 am (0258 GMT) but it was not immediately clear if it was caused by a nuclear test or was a natural quake, Yonhap reported.
South Korea was still trying to confirm whether the North had carried out a test, an official said.
North Korea tested atomic weapons in 2006 and 2009.