Top-level North and South Korean negotiators have talked through the night with no sign of an agreement over ending a military standoff that has threatened to boil over into armed conflict.

After a 10-hour marathon discussion the previous night, the talks passed the 16-hour mark in a second session in the border village of Panmunjom, where the 1950-53 Korean War ceasefire was signed.

The second round was clouded by South Korean claims the North was seeking to influence the negotiating process with provocative military movements.

South Korea's defence ministry said the North doubled its artillery units at the border and deployed two-thirds of its total submarine fleet, about 50 vessels, outside its bases.

"The North is adopting a two-faced stance with the talks going on," said a ministry spokesman who described the scale of the submarine movement as "unprecedented".

At the same time the North deployed naval and artillery units, South Korean and US fighter jets carried out simulated bombing sorties not far from the border.

The negotiations in Panmunjom are being led by South Korean national security adviser Kim Kwan-jin and his North Korean counterpart Hwang Pyong-so, a close confidant of leader Kim Jong-un.

Strained negotiations reflect long history of tension

The gruelling hours reflect the challenge of reaching a compromise, with both militaries on maximum alert and flexing their weaponry across a border that has already seen one exchange of artillery fire.

The roots of the standoff lie in landmine blasts on the border earlier this month that maimed two South Korean soldiers.

Accusing Pyongyang of laying the mines, Seoul retaliated by switching on giant banks of loudspeakers that had lain silent for more than a decade and blasting high-decibel propaganda messages into North Korea.

The North has denied any role in the mine blasts and issued an ultimatum for the South to halt its "psychological warfare" or face attack.

UN Secretary General Ban Ki-moon, a former South Korean foreign minister, urged both sides to "redouble" their efforts to reach a compromise.

The two Koreas have still technically been at war for the past 65 years, as the Korean War ended with a ceasefire that was never ratified by a formal peace treaty.

AFP