Washington (CNN) The man who spearheaded US diplomatic efforts on North Korea until his unexpected retirement earlier this month said the North Koreans were "surprised" that President Donald Trump agreed to meet with leader Kim Jong Un so quickly.

"To be frank with you, I think they were a little bit surprised that Washington, President Trump readily accepted," Ambassador Joseph Yun told CNN. "They thought it would take a little time."

Yun told CNN's Elise Labott in an exclusive interview -- his first since stepping down as US special representative for North Korea policy -- that he welcomes plans for the meeting between Trump and Kim, expected to take place in May.

"I'm very supportive of (Trump's) decision to engage at the highest levels," Yun said. "That is a great outcome."

Last week, Trump accepted an invitation to meet Kim in hopes of securing North Korea's complete denuclearization. The decision follows recent talks between North Korean and South Korean officials around the 2018 Winter Olympic Games in South Korea, which were launched at the start of the year.

Prior to that diplomatic thaw, tensions on the peninsula had hit a peak, with North Korea testing increasingly sophisticated ballistic missiles. Those tests were met with threats of "fire and fury" from the US President and the implementation of harsh new sanctions by the international community.

The tests raised the stakes for talks, according to Yun.

"They now have nuclear weapons and a delivery system that can legitimately threaten all states in the United States," Yun told CNN on Thursday. "That is different from the past, where we were trying to stop them from getting there, so it requires different attention, different focus and a different approach."

Yun served as the US point man on North Korea for 16 months, first under then-President Barack Obama and then under Trump. In that capacity, he conducted a series of back-channel talks with North Korean officials, and traveled to Pyongyang last year, securing the release of imprisoned American student Otto Warmbier, who died shortly after he was freed.

Yun, whose career in diplomacy spanned more than three decades, said he would have loved to bring the leader-level talks forward, and disputes criticism from some analysts that May is too soon for such a high-stakes meeting.

"What I hope comes out of the summit is that President Trump and Kim Jong Un paint a broad brush of the framework of where we need to go, agree on some principles and agree to kick off a process," said Yun. "Process is better than no process, in my view."

Yun acknowledged the North Koreans have wanted a meeting with Trump for some time, but said a lack of agreement within the Trump administration on how aggressively to handle the North Korean nuclear threat prevented talks from happening sooner.

In February, CNN reported a schism within the President's national security team over whether to pursue a so-called "bloody nose" military strike. National security adviser H.R. McMaster is among the advisers said to have advocated for such a strike, while Defense Secretary James Mattis and outgoing Secretary of State Rex Tillerson favored a diplomatic approach.

"In an administration, you're going to have different views," said Yun. "But I think the time has now come, really, to speak with one single unified voice. And that voice has to be that of the President."

Yun said he hopes Mike Pompeo's tenure at the State Department will lead to more cohesion between the agency and the White House on the North Korea issue.

Trump announced on Tuesday he would be replacing Tillerson with Pompeo, who currently serves as director of the CIA.

"I think Director Pompeo -- and I know some of his folks very well -- they have worked very closely with the White House and with the President," Yun said.

Yun added that while he believes the State Department had the White House's support in its broader policy of engagement with North Korea, the upcoming talks make it all the more essential to ensure there is "no gap" in the synchronicity between the President and the diplomats who enact his policy.

Asked explicitly whether Tillerson's ouster, which followed months of personal tensions with the President, would contribute to that improved cohesion, Yun said, "Well, you know, that's a conclusion that I personally draw myself."

"So I would very much hope that going on -- you know we have a very exciting summit scheduled between the President and Kim Jong Un -- that the management will be very tight between the State Department and the White House and that it would lead to good results at the end of the day," he added.

Yun did give credit to Tillerson for helping to create the conditions by which North Korea was ready to come to the table.

"Secretary Tillerson has worked very hard to make sure that our policy, which as you know is maximum pressure and engagement, that there is a good portion of engagement," Yun said. "And I have worked very closely with him to that end."

While Yun thinks expectations for the upcoming meeting need to be lowered, he hopes Trump will be able to convey that the United States has no "hostile intent" toward North Korea, and is only seeking denuclearization.

It's a message Yun emphasized in his own meetings with the North Koreans, with little success.

After Trump accepted Kim's invitation to meet, Yun reached out to North Korean officials at the United Nations, he says, encouraging them to seize the opportunity, and suggested they consider releasing three US citizens imprisoned in their country.

"I pressed the point to them: This would be an incredibly good time for them to release those prisoners so that they can be reunited with their families," said Yun. "And that, in itself, I told them, would be a very positive message."

Yun, who is still easing into retirement, acknowledged he would be tempted to play a role in the upcoming talks if the President asked.

He told CNN he doesn't believe the President will be "played" by the North Korean leader.

"I think the goals are obvious," said Yun, and "a ton of homework" is being done to prepare.

While optimistic, Yun acknowledges the talks could go poorly, increasing tensions on the peninsula even beyond where they stood at the end of 2017.

"I hope it doesn't happen," said Yun, "but obviously if the meeting doesn't go well, that's a possibility."