South Korea said on Wednesday it is considering how to change a decades-old armistice with North Korea into a peace agreement, as U.S. President Donald Trump confirmed a senior American official recently held a top-level meeting with the North Korean leader.

U.S. Secretary of State nominee and CIA Director Mike Pompeo became the most senior U.S. official known to have met North Korean leader Kim Jong-un when he visited Pyongyang.

U.S. officials said Tuesday that the meeting happened in late March, but Trump contradicted that in a tweet early Wednesday.

Mike Pompeo met with Kim Jong Un in North Korea last week. Meeting went very smoothly and a good relationship was formed. Details of Summit are being worked out now. Denuclearization will be a great thing for World, but also for North Korea! —@realDonaldTrump

Pompeo's visit provided the strongest sign yet about Trump's willingness to become the first serving U.S. president ever to meet a North Korean leader.

North Korea and South Korea are also preparing for their own summit between Kim and South Korean President Moon Jae-in, on April 27. A bid to formally end the 1950-53 Korean War is expected to be a major factor in the talks.

"As one of the plans, we are looking at a possibility of shifting the Korean peninsula's armistice to a peace regime," a high-ranking South Korean presidential official told reporters when asked about the North-South summit.

"But that's not a matter than can be resolved between the two Koreas alone. It requires close consultations with other concerned nations, as well as North Korea," the official said.

Trump says U.S. is in contact with North Korea over 3 detained Americans 2:04

At a joint news conference held with Japanese Prime Minister Shinzo Abe on Wednesday night, Trump said that a campaign of maximum pressure on North Korea will continue until Pyongyang sheds its nuclear weapons.

Abe, who is with at Mar-a-Lago in Florida, said there should be "no reward" given to North Korea just because the reclusive country is responding to dialogue over its nuclear program.

Trump again stated that they are working toward the meeting — but that if he doesn't think a meeting with Kim will be successful, he won't go. And if the meeting does happen and isn't fruitful, he'll walk out, Trump told reporters.

South Korea and a U.S.-led UN force are technically still at war with North Korea after the Korean War ended with a truce, not a peace treaty. The U.S.-led United Nations Command, Chinese forces and North Korea signed the 1953 armistice, to which South Korea is not a party.

"I do not know if any joint statement to be reached at the inter-Korean summit would include wording about ending the war, but we certainly hope to be able to include an agreement to end hostile acts between the South and North," the official said.

Talks unthinkable last year

Such discussions between the two Koreas, and between North Korea and the United States, would have been unthinkable at the end of last year, after months of escalating tension, and fear of war, over the North's nuclear and missile programmes.

But then Kim declared in a New Year's speech his country was "a peace-loving and responsible nuclear power" and called for lower military tension and improved ties with the South.

He also sent a delegation to the Winter Olympics in South Korea in February, a visit that began a succession of steps to improve ties.

Pompeo's visit to the North was arranged by South Korean intelligence chief Suh Hoon with his North Korean counterpart, Kim Yong-chol, and was intended to assess whether Kim was prepared to hold serious talks, a U.S. official said. Pompeo flew from a U.S. air force base in Osan, south of Seoul, an official with the South's Defence Ministry said. The South's presidential office declined to comment on the trip.

Amid the diplomatic flurry, CNN reported that Chinese President Xi Jinping also planned to visit Pyongyang soon, after North Korean leader Kim made a surprise trip last month to China, its sole major ally.

South Korean soldiers stand guard in the border truce village of Panmunjom in the Demilitarized Zone (DMZ) dividing the two Koreas. On Wednesday, a high-ranking South Korean presidential official said they are considering renegotiating the peace agreement between North and South Korea. (Jung Yeon-je/AFP/Getty Images)

Speaking in Beijing, Chinese Foreign Ministry spokesperson Hua Chunying said she had no information about any Xi visit to North Korea.

"What I can stress is that China and North Korea have a tradition of high level mutual visits," she told reporters.

"China is willing to strengthen high-level exchanges with North Korea, deepen strategic communications, expand talks and co-operation, and to bring out the important leading role of high-level contact in China-North Korea relations."