Signs of North Korean nuclear test preparations, says South

April 2012 satellite photograph showing North Korea's Punggye-ri nuclear test site. Photograph: AP/GeoEye

North Korea may be preparing to carry out its fourth nuclear test, South Korea's defence ministry said on Tuesday, citing a significant step-up in activity at the North's main test site.

"Our military is currently detecting a lot of activity in and around the Punggye-ri nuclear test site," said Kim Min-Seok, a ministry spokesman.

"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 country's joint chiefs of staff had set up a special task force in case Pyongyang went ahead with an underground detonation.

Barack Obama is due to arrive in South Korea on Friday for a two-day visit and there has been widespread speculation that the North may stage a provocation to coincide with US president's trip.

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, all at Punggye-ri.