The South Korean defence ministry believes that North Korea conducted a nuclear test Tuesday after an earthquake was detected, a media report said.
The report on TV "Channel One" came shortly after a 5.1-magnitude earthquake was detected in North Korea, a potential sign that the country has finally undertaken its third nuclear test, reported Xinhua.
South Korea’s defense ministry estimates the power of the blast at 10 Kiloton.
China's seismology authority measured the temblor at 4.9 magnitude, with a depth of "zero" km. According to Yonhap, the earthquake was detected in North Korea's Kilju county, North Hamkyung province, where a nuclear test site is located.
South Korea military has been immediately put on heightened alert to deter potential cross-border provocations following the North Korean test.
The country's outgoing President Lee Myung-bak is set to convene a national security meeting to discuss the development.
A South Korean official said that North Korea has notified the US and China of its nuclear test plan a day earlier.
North Korea vowed to proceed with missile and nuclear tests targeting the US, its "sworn enemy", after the UN Security Council adopted a resolution to condemn its Dec 12 rocket launch.
Pyongyang conducted similar nuclear tests in 2006 and 2009.