(VOVworld) – The US and South Korea are increasing their monitoring as North Korea has increased activity at its Punggye-ri nuclear test site.

Satellite photograph showing North Korea's Punggye-ri nuclear test site

At a press briefing on Tuesday, South Korea’s Defense Ministry spokesman Kim Min Seok said Seoul has set up a special task force to monitor a possible forth nuclear test. Kim said the military has detected various signs at the Punggye-ri nuclear test site and surrounding areas where North Korea conducted its third nuclear test in February last year. Kim added North Korea is likely to launch a “surprise” nuclear test and refused to provide any further details about the activities.