"Our military is currently detecting a lot of activity in and around the Punggye-ri nuclear test site," ministry spokesman Kim Min-Seok told a press briefing.
"We are thinking of possibilities that the North may stage a surprise nuclear test or just pretend to stage a nuclear test," Kim said, adding that the Joint Chiefs of Staff had set up a special task force in case Pyongyang goes ahead with an underground detonation.
Kim stressed that North Korea's nuclear weapons programme was at a stage where it could conduct a test "at any moment" once the order was given by the leadership in Pyongyang.
North Korea has so far carried out three nuclear tests in 2006, 2009 and 2013.