The North is subject to multiple rounds of UN Security Council sanctions over its atomic and ballistic missile programmes, and has long insisted that its "treasured sword" is not up for negotiation.

But it is willing to halt the programme if its national security -- and that of its leadership -- is guaranteed, the South's national security adviser Chung Eui-yong told a briefing after returning from the North where he met leader Kim Jong Un.

That remains a high threshold -- Pyongyang has considered itself at risk of invasion by the United States since the Korean War ended in a ceasefire in 1953, leaving the two sides technically still at war.

But, Chung said, Kim is willing to discuss denuclearisation in talks with Washington -- which could be the crucial concession needed to enable a dialogue to happen.

The US has long insisted that Pyongyang take concrete steps towards denuclearisation as a precondition.

Tuesday's developments are the latest steps in a rapid Olympics-driven rapprochement on the peninsula, and come after a year of high tensions during which Pyongyang carried out its most powerful nuclear test to date, along with multiple missile launches, including rockets capable of reaching the US mainland.

Kim and US President Donald Trump traded personal insults and threats of war, sending fears of conflict spiralling.

But the Pyeongchang Winter Olympics in the South triggered an apparent transformation, with Kim sending his sister to the opening ceremony and sparking a flurry of cross-border trips as South Korean president Moon Jae-in tries to broker talks between Pyongyang and Washington.

North and South agreed to hold a summit in late April in Panmunjom, the truce village in the DMZ, Chung said after leading the most senior delegation to travel North for more than a decade.

It will be the third meeting between the leaders of North and South, after summits in Pyongyang in 2000 and 2007.

The North "made clear that there is no reason to own nuclear (weapons) if military threats towards the North are cleared and the safety of its regime is guaranteed", Chung said.

Pyongyang "expressed willingness to have frank dialogue with the US to discuss the denuclearisation issue and to normalise North-US relations" he added, and said there would be no provocations such as nuclear or ballistic missile tests while dialogue was under way.

"Also, the North promised not to use atomic weapons or conventional weapons towards the South," he told reporters, adding that Seoul and Pyongyang would set up a hotline between the leaders.

Six party talks, grouping the two Koreas, Russia, China, Japan and the US, and offering the North security and economic benefits in exchange for denuclearisation, broke down almost a decade ago.

- 'Right conditions' -

North Korean state media pictures of the envoys' meeting with Kim in Pyongyang -- which Seoul said lasted more than four hours -- showed the North's leader in a jovial mood, smiling and shaking hands enthusiastically.

"Hearing the intention of President Moon Jae-In for a summit from the special envoy of the south side, he exchanged views and made a satisfactory agreement," the North's official news agency KCNA said earlier.

The North's Rodong Sinmun newspaper, the mouthpiece of the ruling Workers Party, devoted its entire front page Tuesday to the visit under the headline "Comrade Kim Jong Un receives special envoys from the south's president".

The main picture showed Kim with the five South Korean officials in the delegation, and it carried seven other front-page images of the talks.

The Seoul delegation's visit came after the North's leader sent his sister to the Winter Games in the South and invited Moon to a summit in Pyongyang.

Kim Yo Jong's trip was the first to the South by a member of the North's ruling dynasty since the end of the 1950-53 Korean War. Her appearance at the Games' opening ceremony -- where athletes from the two Koreas marched together -- made global headlines.

At the time Moon did not immediately accept the invitation to a summit, saying that the "right conditions" were needed.

The South's envoys will travel to Washington on Wednesday to brief US officials on their discussions in Pyongyang.

It is a challenging task -- Trump has dubbed Kim "Little Rocket Man" and boasted about the size of his nuclear button, while the North Korean leader called the American president a "mentally deranged US dotard".