South Korean President Park Geun-hye demanded on Monday that North Korea apologise over recent landmine blasts, even as the bitter rivals held marathon talks to defuse tensions that have brought the peninsula back to the brink of armed conflict.

Park said anti-North propaganda broadcasts would continue unless Pyongyang took responsibility for landmine explosions early this month that wounded two South Korean soldiers in the Demilitarised Zone (DMZ) separating the two countries.

North Korea denies it laid the mines. Seoul and Pyongyang have remained technically in a state of war since the 1950-53 Korean war ended in a truce, rather than a peace treaty.

The landmine blasts escalated into a crisis that saw both sides exchange artillery fire on Thursday and ramp up their military readiness. The United Nations, the United States and the North's lone major ally, China, have all called for calm.

"We need a clear apology and measures to prevent a recurrence of these provocations and tense situations," Park told a meeting with her top aides, according to a statement released by her office. "Otherwise, this government will take appropriate steps and continue loudspeaker broadcasts."

Seoul and Washington were reviewing the possibility of bringing in "strategic" US military assets, South Korean Defence Ministry spokesman Kim Min-seok said, without elaborating.

North Korea threatened military action in response to annual exercises by US and South Korean forces two years ago, leading to a standoff during which US stealth bombers flew over South Korea and an aircraft carrier was sent to the area.

"Our position at this point is to deter the North's provocation," Kim told a news briefing. "But if they wage provocation, our response will be merciless and they will truly feel sorry."

Reclusive North Korea had deployed twice the usual artillery strength at the border and had around 50 submarines away from base, the South's defence ministry said.

North Korea's state media has also kept up its anti-South rhetoric as the inter-Korean talks continued at the Panmunjom truce village inside the DMZ. Its KCNA news agency said 1 million young people had volunteered to join or rejoin the army – an assertion impossible to verify because of the North's isolation.

Park cited a story on Monday that two South Korean soldiers had delayed their discharges and South Korea's army said about 50 soldiers had taken the same step. Park's approval rating rose to 41 per cent in a Realmeter poll conducted last week.

"I think that kind of patriotism can protect our country, setting an example for young people," she said.

Marathon talks

The negotiations began on Saturday evening, shortly after North Korea's deadline passed for Seoul to halt the anti-Pyongyang propaganda broadcasts or face military action. They broke up before dawn on Sunday and restarted that afternoon.

The delegates were "continuing talks for long hours in the midst of the critical situation" on the peninsula, Min Kyung-wook, a spokesman for South Korea's presidential Blue House, told reporters, without giving details.

Chung Young-chul, a North Korea expert at Sogang University's Graduate School of Public Policy in Seoul, said Park's strong words may indicate a lack of progress, although other observers said the unusual length of the talks bodes well.

"I am not really optimistic about the talks because they both have heavy demands that can't be dropped," Chung said.

"It seems difficult to get any agreement and I think they are locking horns and tension will persist for a while."

Park's national security adviser, Kim Kwan-jin, and Unification Minister Hong Yong-pyo are representing the South in the talks. Hwang Pyong So, the top military aide to the North's leader, Kim Jong Un, and Kim Yang Gon, a veteran North Korean official in inter-Korean affairs, are representing Pyongyang.

Ties have been virtually frozen since the 2010 sinking of a South Korean warship, which Seoul has blamed on a North Korean submarine. Pyongyang denies responsibility.

Days after the landmine incident, Seoul began its propaganda broadcasts in random three-hour bursts from 11 banks of loudspeakers, including news reports and K-pop music from the South, resuming a tactic both sides halted in 2004.

The crisis escalated on Thursday when the North fired four shells into the South, according to Seoul, which responded with a barrage of 29 artillery rounds. North Korea declared a "quasi-state of war" in front-line areas and set an ultimatum for Seoul to halt its broadcasts.

That deadline passed on Saturday without incident.

The United States, which has 28,500 soldiers based in South Korea, is conducting annual military exercises with the South. North Korea regularly condemns the maneuvers as preparation for war.