The South Korean government said Tuesday that heightened activity has been detected at North Korea's underground nuclear test site, indicating possible preparations for another atomic test, Seoul's Yonhap News Agency reported."A lot of activity is currently being seen, so our forces are keeping in mind the possibility that North Korea may suddenly conduct a nuclear test in a short period of time, or as in previous cases, deceive us with what appears to be a nuclear test," Defense Ministry spokesman Kim Min-seok was quoted as saying at a press briefing in Seoul.North Korea appears to be ready to carry out a nuclear test at any time if it wishes, and the communist nation has recently made remarks claiming to be preparing something "unimaginable" for its enemies and "something big" before April 30, Kim added. A crisis management task force was set up between the defense ministry and the Joint Chiefs of Staff on Monday to prepare for any eventualities.North Korea has been watched closely for signs of another nuclear test after it threatened last month to conduct a "new form" of test, which experts say could mean the use of a uranium-based device.North Korea's three previous nuclear tests in 2006, 2009 and 2013 are believed to have used plutonium. The increased activity could have greater repercussions as it comes ahead of US President Barack Obama's official two-day visit to Seoul that will start Friday, the report said.