SEOUL - The South Korean Defense Ministry believed that the Democratic People's Republic of Korea (DPRK) has conducted a nuclear test on Tuesday, a local TV station reported.

The report from local TV "Channel One" came shortly after a 5.1-magnitude earthquake was detected in the DPRK, a potential sign that the country has finally undertaken its third nuclear test.

An official from the DPRK Foreign Ministry told Xinhua over the phone that he had no knowledge of the reported seismic activity.

China's Earthquake Networks Center measured the temblor at 4.9 magnitude, with a depth of "zero" km.

The South Korean Defense Ministry estimated the likely nuclear test at 6 to 7 kilotons, while an earlier report from the Yonhap news agency said the ministry calculated the explosion at 10 kilotons.

According to Yonhap, the earthquake was detected at 11:57 local time (0257 GMT) in the DPRK's Kilju County, North Hamkyung Province, where a nuclear test site is located.

The South Korea military has been immediately put on heightened alert to deter potential cross-border provocations following the reported DPRK test.

Meanwhile, the country's outgoing President Lee Myung-bak has convened a national security meeting to discuss the development.

A South Korean official said the DPRK has notified the United States and China of its nuclear test plan a day earlier.

Diplomatic sources said the UN Security Council will meet over the possible test on Tuesday morning.

The DPRK vowed to proceed with missile and nuclear tests targeting the United States, its "sworn enemy," after the UN Security Council adopted a resolution to condemn its December 12 rocket launch.

Pyongyang conducted similar nuclear tests in 2006 and 2009.