Satellite imagery of the Punggye-ri nuclear test facility in North Korea reveals signs that indicate Pyongyang may be preparing to test a nuclear device.
A government source in Seoul told the Yonhap news agency on condition of anonymity that North Korea may conduct a test "at any moment."
"There was increased activity by workers and vehicles near the entrance tunnel at the Punggye-ri test site," the source was quoted as saying.
North Korea in December launched a long-range rocket in an attempt to orbit a satellite. Similar launches in 2006 and 2009 coincided with nuclear tests. A launch attempt last April failed.
Seoul said it has increased military monitoring in response to the latest nuclear concerns.
South Korean President Lee Myung-bak, meanwhile, hailed the successful launch Wednesday of Korea Space Launch Vehicle-1. The rocket deployed a satellite into orbit.