North Korea military
Soldiers march past the podium during a military parade in Pyongyang (Reuters)

Military tensions between North and South Korea are mounting again as Pyongyang reportedly ordered frontline troops "to be ready for war" ahead of a nuclear test.

North Korean dictator Kim Jong-un has placed the country under martial law on the advice of top brass, South Korea's Joongang Daily reported.

Kim has also decided to bring forward a nuclear test Pyongyang pledged to carry out in retaliation for tighter sanctions by the UN Security Council.

Analysts had believed that the nuclear test would go ahead on 16 February as part of the celebrations to commemorate the birthday of Kim's late father, Kim Jong-il.

An alternative was mooted as 25 February, when South Korea's newly elected president, Park Geun-hye, is sworn in.

But the test will now take place within the next five days, according to the Joongang Daily.

South Korean defence minister Kim Kwan-jin told troops deployed on the border with the North that they should be ready to cope swiftly with any "provocations".

"Provocations are always possible because North Korean troops have been put on full alert," Kim told AFP.

Outgoing South Korean president Lee Myung-Bak held an emergency security meeting of senior ministers and military staff and warned Pyongyang against "reckless action".

"The government urges North Korea to immediately stop all provocative statements and actions and abide by international obligations including UN Security Council resolutions," he said.

"If the North miscalculates and commits a provocative act again, it would face various serious consequences."

The UN Security Council unanimously extended existing sanctions on North Korea earlier in January over a long-range rocket launch by Pyongyang in December.

North Korea had been banned from missile tests after it conducted nuclear tests in 2006 and 2009.

Kim Jong-un
North Korean leader Kim Jong-un speaks in Pyongyang (Reuters) Reuters