Washington (CNN) As top national security officials continue to work behind the scenes to prepare for an audacious one-on-one meeting between President Donald Trump and Kim Jong Un, a key sticking point has emerged: where to hold the talks.

Even as matters of substance like the scope of the summit and the composition of the delegations are being sorted through, the two sides remain at odds over the logistical details, according to US and other officials familiar with the matter.

During CIA Director Mike Pompeo's secret Easter weekend visit to North Korea, Kim personally promised that denuclearization would be a topic of discussion during the meeting with Trump, two sources familiar with the meeting told CNN. Another administration official familiar with the talks described Kim as "personable and well prepared" for the discussion.

But the venue for the talks remains the main sticking point, that official and several others familiar with the matter said.

Pompeo's visit to Pyongyang, according to Trump in a tweet Wednesday morning, "went very smoothly." He added that "denuclearization will be a great thing for the world, but also for North Korea!" and "details of Summit are being worked out now."

At Mar-a-Lago later Wednesday, Trump hailed Pompeo's personal chemistry with the North Korean despot.

"He just left North Korea, had a great meeting with Kim Jong Un and got along with him really well, really great," Trump said. "And he's that kind of a guy. He's really smart but he gets along with people."

Possible locations

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US officials have narrowed possible locations for the historic talks to a handful of sites in Asia and Europe, according to two people familiar with the matter. Those officials said that previously discussed sites -- including Washington, Beijing, Seoul and Pyongyang -- have either been ruled out or are now considered less likely.

Trump told reporters on Tuesday that there were five locations under consideration, none of which are in the United States.

"The big things North Korea will be looking for are countries that are at least somewhat friendly to them -- these are increasingly few and far between -- and a location that Kim Jong Un can quickly and safely travel to," says Jenna Gibson, director of communications at the Korea Economic Institute.

Likewise, she says, "we... know that President Donald Trump hates travel, especially long trips. He has even cut trips short before in an effort to get home quickly. He will want to make this meeting happen in a location that is as physically close to the U.S. as possible. He is also very aware of optics -- he knows he can't give away too much before the meeting even begins by agreeing to meet in Pyongyang, Beijing or Moscow."

The Mongolian capital of Ulaanbaatar "makes a lot of sense," she says.

"They have good historical relations with North Korea -- Mongolia was the second country to formally recognize the DPRK, and they provided some help to the North during the Korean War. Mongolia now prides itself on its neutrality, and has good relations with both North Korea and the United States."

Asian destinations

In Southeast Asia, Singapore has been viewed as a likely possibility, with cities in Vietnam and Thailand also under consideration.

Singapore has had diplomatic relations with North Korea since the 1970s, said Jasper Kim, director of the Center for Conflict Management at Ewha University in Seoul, although recently relations have been strained.

"Singapore represents to both parties an idyllic economic city state -- something that North Korea, or a free economic zone within the country could aspire to."

He considers Vietnam and Thailand to be less likely venues.

"Vietnam, symbolically, would be an interesting choice. It was a divided state, what you see after the post-divided state era is rapid growth, but it still holds a lot of the hallmarks of the communist era. Holding it there would be a symbol from Kim -- 'we can keep communist political system married to a Western capitalist system.'"

He says Thailand, currently under military dictatorship, would be a "comforting" choice for a North Korean delegation, "but probably not up high on the list. It could be a third, neutral choice," if the two sides can't agree on another venue.

US officials say the location for the talks is subject to a number of factors, including how far Kim is willing or able to travel by train or airplane.

This is another reason the Mongolian capital, Ulaanbaatar, is also seen as a possibility, though it's not considered as likely as the other locations.

"This location would definitely be advantageous to North Korea. It's a state that's had relations with North Korea for half a century. At the same time it's a burgeoning economy, which sends a signal -- that this could be North Korea (in the future)," Jasper Kim said.

"(Kim) doesn't like to travel far -- it is definitely a factor. Politically, economically and from a security point of view it has upsides for North Korea."

However, if a location in Southeast Asia is chosen, it will be "because all of the other options are ruled out for some reason or another," says Gibson.

"None of the potential Southeast Asian locations are particularly easy for either side to get to, and they don't have the same diplomatic or historical ties to the issue that some of the other locations have."

European options

Switzerland and Sweden are also possibilities, along with other European countries that maintain diplomatic relations with North Korea.

Stockholm would be on the "upper half" of a list of likely venues, Kim said. "Among Western states, Sweden is arguably among the closest (to North Korea). That kind of diplomatic relationship is important."

Switzerland would also be a top choice, he said. "Kim went to school there, he's familiar with it. The US is familiar with it as well and it has a long history as a neutral state. It's way up there on the list -- it sits in the intersection of the Venn diagram of neutrality (and) security."

Gibson agrees.

"(Switzerland) has earned a reputation for neutrality, which is one of the key things the two sides will be looking for.

"Kim Jong Un spent time there as a student, which also means he will be more comfortable traveling there than any other Western states. Finally, the North Koreans are comfortable working through the UN system -- the large UN presence in Geneva may ease concerns about prep work and logistics. And it's a lot closer for President Trump than many of the other possible locations that have been floated."

Neutral venues

More pressing, the officials said, is managing the optics of a location that appears neutral to both sides. Kim, for example, is believed to be unwilling to meet on a US aircraft carrier off the Korean coast. Trump, meanwhile, may be unwilling to travel to the demilitarized zone lest he be viewed as arriving on Kim's home turf.

One US official said the North Korean capital, Pyongyang, is not an option for meeting, in the view of officials, who described it as all but impossible to secure and protect from bugs and nerve agents.

As long as a site for the talks remains undecided, officials have been unable to identify a date for the event. Trump and the White House maintain they are aiming for a summit by late May or early June.

Secret talks

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Secret talks between North Korean and US intelligence officials have been ongoing for weeks, as CNN has previously reported. South Korean intelligence officials have a long history of working very closely with US counterparts; there is one US official paired to nearly every official working in South Korean intelligence and they often work side by side -- in Seoul and other areas like Afghanistan.

National security adviser John Bolton, in just 10 days on the job, has already met with his counterparts from South Korea and Japan, two sources familiar with the meetings told CNN. He met both national security advisers on the same day last week, and was received fondly.

It was a "very substantive meeting," according to Matt Pottinger, the top Asia official on the National Security Council, who said Bolton's talks included a "very clear exchange of views on planning for both of these summits."

It's possible Bolton and Pompeo will return to Pyongyang before the summit takes place, the sources familiar with the meetings said.

Additionally, a "hotline" between the two Korean leaders is being set up this week, a topic discussed by South Korean Cultural Minister Do Jong-hwan on Tuesday. According to a source familiar with the planning, arranging a hotline is difficult technically and otherwise, and officials are still deciding where to install it -- -- inside the South Korean President's office or a nearby agency.

The cultural minister also told reporters in Seoul that South Korea will be seeking a peace treaty during those discussions that would formally end the Korean War, which technically is still underway even though hostilities ended more than six decades ago.