WASHINGTON/SEOUL (Reuters) - U.S. President Donald Trump said on Wednesday that preparations were advancing for a first-ever summit with North Korea after CIA director Mike Pompeo visited Pyongyang and formed a “good relationship” with its leader Kim Jong Un.

Pompeo, who Trump has nominated to become his next secretary of state, became the first U.S. official known to have met Kim when he flew to Pyongyang to lay the groundwork the planned summit in which the U.S. president hopes to persuade North Korea to give up its nuclear weapons.

“Mike Pompeo met with Kim Jong Un in North Korea,” Trump tweeted. “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!”

Trump said the meeting took place last week, but U.S. officials said Pompeo actually visited over the Easter weekend, which ran from March 31 to April 2.

The trip, which followed a North Korean invitation, was first broached through intelligence channels that his staff was using with North Korea’s spy service, a U.S. official said.

It provides the strongest sign yet of Trump’s willingness to become the first serving U.S. president to meet a North Korean leader as he seeks to resolve a crisis over Pyongyang’s development of nuclear missiles capable of hitting the United States.

Trump said on Tuesday he believed there was a lot of goodwill in the diplomatic push, but added it was possible the summit - which the president had earlier said could take place in late May or early June - may not happen, in which case the United States and its allies would maintain pressure on North Korea through sanctions.

U.S. officials said the visit by Pompeo, one of Trump’s most trusted advisers, 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 about giving up his nuclear weapons.

They said Pompeo’s conversations in Pyongyang fueled Trump’s belief that productive negotiations were possible, but far from guaranteed. They said no site had yet been chosen for a summit.

Pompeo flew to North Korea from a U.S. air force base in Osan, south of Seoul, an official with South Korea’s defense ministry said.

Pompeo told his Senate confirmation hearing for the post of secretary of state last week he was optimistic a course could be set at a Trump-Kim summit for a diplomatic outcome with North Korea, but added that no one was under any illusion that a comprehensive deal could be reached at that meeting.

Pompeo said the aim would be “an agreement...such that the North Korean leadership will step away from its efforts to hold America at risk with nuclear weapons” and that Pyongyang should not expect rewards until it takes irreversible steps.

NORTH-SOUTH SUMMIT

News of Pompeo’s trip came as South Korean President Moon Jae-in was preparing for his own summit with Kim, on April 27, with a bid to formally end the 1950-53 Korean War a major factor.

“As one of the plans, we are looking at a possibility of shifting the Korean peninsula’s armistice to a peace regime,” a top South Korean presidential official told reporters in Seoul.

“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.”

South Korea and a U.S.-led U.N. force have remained technically at war with North Korea since 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.

FILE PHOTO: A combination photo shows CIA Director Mike Pompeo (L) in Washington, North Korean leader Kim Jong Un (C) in Pyongyang, North Korea and U.S. President Donald Trump (R), in Palm Beach, Florida, U.S., respectively from Reuters files. REUTERS/Yuri Gripas (L) & KCNA handout via Reuters & Kevin Lamarque (R)

The South Korean official said he did not know if any joint summit statement 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.”

Trump said on Tuesday he backed efforts between North and South Korea aimed at ending the state of war.

Such discussions between the two Koreas, and between North Korea and the United States, would have been unthinkable last year when North Korea conducted repeated missile tests, detonated its largest ever nuclear bomb, and said the United States was now within range of its weapons.

The tests and escalating rhetoric between Trump and Kim raised fears of war before the North Korean leader called, in a New Year speech, for lower military tensions and improved ties with South Korea.

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

Despite China’s traditional friendship with North Korea, Beijing has been angered by Kim’s weapons development and backed successive rounds of U.N. economic sanctions, from which Pyongyang is seeking respite.On Wednesday, finance ministers from the Group of Seven industrialized countries issued a statement saying they were concerned about North Korea’s evasion of sanctions and its “ability to access the international financial system.”