"North Korea will not be able to avoid being held accountable for (the test)," national security adviser Chun Yung-Woo told reporters after an emergency security meeting presided over by President Lee Myung-bak.
The nuclear test, which North Korea claimed was successful, was carried out at 11.57 a.m. in North Korea's Kilju county, North Hamkyung province.
An artificial earthquake with a magnitude of 4.9 was detected from the town, where the North Korean nuclear test facility is located.
South Korea's military was immediately put on heightened alert to deter potential cross-border provocations, while the South Korean and US forces upgraded their border surveillance level.
The South Korean defence ministry is keeping an eye on developments in North Korea to monitor potential signs of additional nuclear or missile tests.
Seoul: The South Korean government condemned the nuclear test by North Korea Tuesday, calling it a "clear violation" of UN resolutions. The South Korean military was put on high alert.
First Published: Tuesday, February 12, 2013, 13:00