The North is open to "frank" talks with the United States on denuclearisation and would suspend missile and nuclear tests while dialogue was underway, Chung Eui-yong said on Tuesday after returning from a meeting in Pyongyang with leader Kim Jong-un.

US President Donald Trump gave a cautious welcome to the announcement while Vice President Mike Pence said Washington would keep "maximum pressure" on Pyongyang until it takes concrete steps toward denuclearisation.

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"Possible progress being made in talks with North Korea," Mr Trump tweeted.

"For the first time in many years, a serious effort is being made by all parties concerned.

"The World is watching and waiting! May be false hope, but the U.S. is ready to go hard in either direction!"

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In the Oval Office, the president said statements from the Koreas were "very positive".

"They seem to be acting positively but we are going to see," Mr Trump said.

"The statements coming out of South Korea and North Korea have been very positive.

"We have come a long way at least rhetorically with North Korea. It would be a great thing for the world, it would be a great thing for North Korea, it would be a great thing for the peninsula, but we will see what happens.

"We are going to do something, one way or the other we are going to do something and not let that situation fester."

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

But it is willing to abandon the programs if its national security - and that of its leadership - is guaranteed, Mr Chung said.

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 technically still at war.

But Mr Chung said Mr Kim was willing to discuss denuclearisation with Washington - a crucial concession that could enable a dialogue.

Washington has long insisted Pyongyang take concrete steps towards denuclearisation as a precondition.

"Whichever direction talks with North Korea go, we will be firm in our resolve," Mr Pence said.

"The United States and our allies remain committed to applying maximum pressure on the Kim regime to end their nuclear program," he said.

"All options are on the table and our posture toward the regime will not change until we see credible, verifiable, and concrete steps toward denuclearisation."

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Olympics-driven rapprochement

Tuesday's developments are the latest steps in a rapid Olympics-driven rapprochement on the peninsula.

They follow a year of high tensions during which Pyongyang carried out its most powerful nuclear test to date, along with launches of rockets capable of reaching the US mainland.

Mr Trump has dubbed Mr 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".

But the Pyeongchang Winter Olympics in the South triggered an apparent transformation, with Mr Kim sending his sister to the opening ceremony, 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, Mr 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, but the first to take place in the DMZ 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", Mr 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 underway.

"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.

Mr Kim also said he would "understand" if the South goes ahead with delayed joint military exercises with the US that usually infuriate Pyongyang, a senior official at the South's presidential office added.

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'Very important breakthrough'

The visit produced "a very important breakthrough", Cheong Seong Chang of Sejong Institute think tank said, calling the results "an important first step towards stably managing the North's nuclear and missile threats, preventing war on the Korean peninsula and building political and military trust".

He cautioned that the definition of "military threats" the North wanted to see removed was "up for interpretation" but said he believed Washington and Pyongyang "would soon begin serious dialogue".

Previous negotiations have ultimately foundered, however. 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.

North Korean state media pictures of the delegation's extended meeting with Mr Kim in Pyongyang 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.

Rodong Sinmun newspaper, the mouthpiece of the ruling Workers Party, devoted its entire front page to the visit.

The trip came after the North's leader sent his sister, Kim Yo Jong, to the Winter Games, the first visit to the South by a member of the North's ruling dynasty since the end of the 1950-53 Korean War.

Mr Kim also invited Mr Moon to a summit in Pyongyang but the South Korean leader said the "right conditions" were needed.