SEOUL (Reuters) - Top aides to the leaders of North and South Korea negotiated overnight and into Monday morning in a marathon bid to try to ease tensions involving an exchange of artillery fire that has brought the peninsula to the brink of armed conflict.

A South Korean army's Multiple Launch Rocket System (MLRS) (R) is deployed just south of the demilitarized zone separating the two Koreas in Yeoncheon, South Korea, August 23, 2015. REUTERS/Lim Byung-sik/Yonhap

The rare and unusually long meeting at the Panmunjom truce village inside the Demilitarized Zone (DMZ) began on Saturday evening, shortly after North Korea’s deadline for Seoul to halt anti-Pyongyang propaganda broadcasts or face military action.

It broke up before dawn on Sunday and restarted that afternoon with the rivals on high military alert. The talks were continuing past dawn on Monday morning, an official at South Korea’s presidential Blue House told Reuters soon after 7 a.m. EDT.

The North had deployed twice the usual artillery strength at the border and had more than 50 submarines away from base, the South’s defense ministry said on Sunday.

South Korea, also on high alert, has said it had no plans to halt the propaganda broadcasts that triggered the latest standoff.

The envoys, shown on TV exchanging handshakes and tight smiles at the start of their meeting on Saturday, discussed ways to resolve tension and improve ties, the Blue House said in a brief statement early on Sunday.

“Both sides are under big pressure to get something out of this,” said Jeon Young-sun, professor at the Institute of the Humanities for Unification at Konkuk University in Seoul.

The talks took place in South Korea’s Peace House, just south of Panmunjom’s often-photographed sky-blue huts, and the same venue where lower-level talks between the bitter rivals took place in February 2014, without agreement.

The negotiating session that began on Saturday was interrupted with breaks for both sides to consult with their respective governments, and for snacks, the South’s Yonhap News Agency reported.

“North Korea wants to stop broadcasts, while South Korea can’t do it without achieving anything back,” Jeon said.

“QUASI-STATE OF WAR”

North Korea and South Korea have remained technically in a state of war since their 1950-53 conflict ended in a truce, not a peace treaty, and inter-Korean relations have been in a deep freeze since the deadly 2010 sinking of a South Korean warship. Pyongyang denied responsibility.

The current tensions began early this month when two South Korean soldiers were wounded by landmines along the border. The North denies laying the mines. Days later, 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 any reported incident.

The United Nations, the United States and the North’s lone major ally, China, have all called for calm.

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

South Korean President Park Geun-hye’s national security adviser, Kim Kwan-jin, and Unification Minister, Hong Yong-pyo, met with 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, on Saturday, prompting hopes for a breakthrough.

Pyongyang’s two negotiators made an unexpected visit to the South last October to attend the closing ceremony of the Asian Games, where they met Kim Kwan-jin. Those talks raised hopes for an improvement in relations, but that did not materialize.

North Korea has been hit with U.N. and U.S. sanctions because of repeated nuclear and missile tests, moves that Pyongyang sees as an attack on its sovereign right to defend itself.