South Korea says it will not stop blasting high-decibel propaganda messages into North Korea until Pyongyang offers a "clear apology" over a series of alleged provocations.

Key points: South Korea won't stop broadcasting propaganda across the border unless North offers apology

South Korea won't stop broadcasting propaganda across the border unless North offers apology Both sides are shoring up defences near border

Both sides are shoring up defences near border Talks between high-level at border truce village are continuing

The remarks from South Korean president Park Geun-hye came even as top-level representatives from both countries engaged in marathon talks which began Saturday in the border truce village of Panmunjom, where the 1950-53 Korean War ceasefire was signed.

But hope over the outcome of the latest talks was clouded by South Korean claims that the North was seeking to influence the negotiating process with provocative military movements, including moving additional artillery units to the border and deploying dozens of submarines.

"[North Korea] should make a clear apology ... and ensure that there will be no further provocations," Ms Park said in a speech, blaming Pyongyang for sparking the current military crisis with "provocative activities".

The South's defence ministry, meanwhile, said the North had doubled its artillery units at the border and deployed two-thirds of its total submarine fleet — around 50 vessels — outside their bases.

The ministry said it was closely monitoring the movement of North Korean landing craft, following a report by the Yonhap news agency that the North has deployed about 10 air-cushioned amphibious landing craft carrying special forces to a frontline naval base.

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

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.

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.

"Any resolution of issues now on the table will require a bold decision from their leaders," said Yang Moo-Jin, a professor at the University of North Korean Studies in Seoul.

"The fact that they are still talking shows a genuine determination to get something out of this — the only question is what."

While the North moves around submarines and artillery units, South Korean and US fighter jets have been carrying out simulated bombing sorties not far from the border.

Landmines and loudspeakers at centre of contention

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

Accusing Pyongyang of laying the mines, Seoul retaliated by switching on giant banks of propaganda loudspeakers that had lain silent for more than a decade.

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

Analysts said the North would never apologise for the mine blasts, while South Korea would reject any compromise that might be seen to reward Pyongyang's belligerence.

"The two sides may be able to come up with a statement in which some sort of 'regret' is expressed without explicitly naming the North," said Jeung Young-tae, an analyst at the Korea Institute for National Unification in Seoul.

"But I don't think such a vague statement will work this time," Mr Jeung said, stressing that the case of the maimed soldiers — both of whom lost legs — had become an emotional issue in the South.

"I think the best outcome of this meeting will be an agreement for another high-level meeting in the future, such as defence ministerial talks," he added.

That would still leave open the issue of the propaganda broadcasts, which Seoul has vowed to continue.

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

Technically, the two Koreas have 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.

There are nearly 30,000 US troops permanently stationed in South Korea, and the US military's top officer on Saturday reiterated Washington's commitment to the defence of its ally.

South Korea's defence ministry said Seoul and Washington would discuss the possible deployment of strategic US military assets on the peninsula.

Despite the uncertainty, the crisis has failed to generate any real panic among ordinary South Koreans, who have become largely used to the North's regular threats of imminent war.

However, thousands of civilians living on frontline border islands or near military propaganda units have spent much of the past three days in underground shelters as a preventive measure.

The South Korean embassy in Beijing issued a rare travel warning on Monday, advising its citizens not to travel to regions in China close to the North Korean border.

AFP