North Korea nuclear test
A North Korean flag on a tower flutters in the wind at a North Korean village near the truce village of Panmunjom in the demilitarized zone separating the two Koreas in this picture taken just south of the border, in Paju. - Reuters

The US and South Korea are closely monitoring North Korea's nuclear test sites over speculation that the North could conduct another nuclear test.

South Korea has confirmed that the North may carry out an "additional nuclear test" which recent reports suggested as imminent.

"The military considers there is still a threat of an additional nuclear test and possibility of a military provocation by the North. South Korea and the US forces are intensively operating their intelligence assets to closely monitor the North Korean military's activities," army official Colonel Wi Yong-seop told a press conference in Seoul.

Soon after the "safe and perfect" third nuclear test, North Korea had warned that it would take more such steps, defying the international outcry.

Only days back, hectic activity at one of the test sites was reported. Earlier reports suggested that Pyongyang might be preparing to conduct two nuclear tests in quick succession.

The South Korean military has been put on high alert over the situation.

Apart from the increased surveillance, South Korea and the US are also gearing up to conduct a six-day joint-naval drill in the East Sea to test their combat readiness.

American P-3 maritime patrol aircraft, submarines and destroyers belonging to the South will be taking part in the drill, according to an official statement.

The South Korean government has also issued a "warning" status against any cyber attack ahead of the swearing-in ceremony of president-elect Park Geun-hye on 25 February.

Experts in Seoul have stepped up security measures which include steps to guarding the country's GPS systems.

Many ministries including maritime affairs, agriculture, and defence have also beefed up security.