Washington (CNN) President Donald Trump's rosy outlook at the prospect of meeting with North Korean despot Kim Jong Un is about to hit a wall of hard truths erected by US allies, outside experts and officials within his administration.

Trump reacted to Kim's surprise meeting with Chinese President Xi Jinping early on Wednesday via Twitter, saying chances of denuclearization had risen from "not even a small possibility" to a "good chance."

But the China meeting only solidified the sense of uncertainty pervading US efforts to organize in a matter of months the highest-stakes US-North Korean talks ever, as senior officials in Washington admitted they did not know until Tuesday afternoon whether Kim was indeed in China. And a key US ally -- Japan -- is preparing efforts to warn Trump of potential traps in meeting Kim.

"For years and through many administrations, everyone said that peace and the denuclearization of the Korean Peninsula was not even a small possibility. Now there is a good chance that Kim Jong Un will do what is right for his people and for humanity," Trump tweeted. "Look forward to our meeting!"

For years and through many administrations, everyone said that peace and the denuclearization of the Korean Peninsula was not even a small possibility. Now there is a good chance that Kim Jong Un will do what is right for his people and for humanity. Look forward to our meeting!

Privately, Trump has made clear to advisers that he wants the meeting to happen, expressing few reservations about the prospects of a face-to-face meeting with Kim, a source familiar with the ongoing negotiations said. But in the coming weeks, US officials and at least one key US ally will look to dampen that optimism.

Japanese Prime Minister Shinzo Abe will come bearing a list of concerns over Trump's face-to-face with Kim when he arrives in the US next month to meet with the President, a person familiar with the Japanese efforts said. The meeting -- which could occur at Trump's Mar-a-Lago estate in Florida after he returns from a trip to South America -- came at Abe's insistence after learning that Trump had accepted an invitation to meet with Kim.

Meanwhile, some officials inside Trump's administration have continued to privately cast doubts that a meeting with Kim will ever materialize, even as Trump himself has pressured his aides to make headway on organizing the historic encounter.

Just this month -- days before Trump quickly accepted North Korea's invitation to meet -- senior administration officials told reporters the US would not hold direct talks until North Korea takes "concrete steps" toward denuclearization. That condition has since been discarded, but now those officials are working to ensure Trump does not walk into his meeting with Kim with unduly high expectations.

"I wouldn't say optimism is called for right now. I would be very cautious because ... what North Korea expects out of this summit and what the US expect may not be potentially aligned," said Sue Mi Terry, a former CIA analyst and North Korea expert. "Optimism is the last word I would use for this."

US planning

The planning for a summit has included Mike Pompeo, the director of the CIA whom Trump has tapped to become secretary of state. That shakeup was announced on the heels of the diplomatic breakthrough, and senior administration officials attributed the change to Trump's closer alignment with Pompeo on the issue.

As the White House works to secure his confirmation, Pompeo and a team at the CIA have been working through intelligence backchannels to make preparations for the Kim talks. Meanwhile, officials at the State Department -- led by Marc Knapper, the chargé d'affaires in Seoul, and Susan Thornton, the assistant secretary of State for East Asian and Pacific affairs -- have been working separately to prepare for the summit.

That's included the logistical work that goes into any presidential travel abroad, including potential locations and necessary staffing. It's also encompassed a thorough accounting of past attempts at talks with the North Koreans in the hopes of identifying potential pitfalls.

The efforts have fed into a working group convened by the National Security Council's top Asia hand, Matthew Pottinger. But the leadership of the NSC is due to change soon. Trump announced last week he was shoving aside his national security adviser H.R. McMaster for John Bolton, the arch-hawk who has in the past advocated military action against North Korea.

In appearances on Fox News, his former employer, Bolton has praised Trump's willingness to meet Kim, even suggesting the summit be held as soon as the end of March. But he's also warned against drawing out negotiations, and has suggested the Trump-Kim meeting be cut short if the President determines North Korea isn't serious about denuclearization.

With Bolton and Pompeo at Trump's side, experts have said the US may take a harder line against North Korea -- which could be newly emboldened by its dialogue with China.

A shifting timeline for the talks has reflected the massive diplomatic undertaking. When South Korea's national security adviser originally announced Trump's willingness to meet with Kim, he said the President intended for them to occur "by May." A week later, the White House said in a description of a phone call Trump held with the South Korean President that the talks were on track to happen "by the end of May." On Tuesday, press secretary Sarah Sanders declined to offer a deadline at all, saying only that "no time or date" had been set.

Many unknowns

US officials say the talks will most likely take place in late May -- or perhaps even June -- should they occur. It's also entirely unclear where the meeting will be held. Officials and analysts have said the Demilitarized Zone on the Korean border is a possibility, along with neutral sites in Europe like Sweden or Switzerland, or even remote capitals, such as Ulan Bator in Mongolia. Trump is not fond of the logistics of foreign travel, such as sleeping in foreign hotels, people close to him have said. And Kim had never left North Korea as leader before this week.

The White House has declined to say whether official contact has yet been established between North Korea and Washington, which would allow US officials to confirm whether Kim had indeed vowed to halt missile and nuclear testing ahead of talks. In the absence of that confirmation, Trump and his aides have relied partly on the characterizations of the South Koreans, who came bearing the invitation earlier this month, and the Chinese, who provided a briefing to the White House on Tuesday after Kim and President Xi Jinping met in Beijing.

According to Chinese state media, Kim told Xi he was open to summit talks with Trump. But the North Koreans have not themselves confirmed Kim's intent to meet with Trump.

"If South Korea and the United States respond with good will to our efforts and create an atmosphere of peace and stability, and take phased, synchronized measures to achieve peace, the issue of the denuclearization of the peninsula can reach resolution," Kim said, according to Xinhua.

Some US officials saw Kim's surprise trip to China as a sign of improving odds that talks between Trump and the North Korean leader will eventually come to pass, saying Kim appears willing to engage in diplomacy. But others saw the trip -- which occurred with little US involvement -- as an indication that Beijing was exerting itself in the proceedings.

US officials said they got no advance warning from the Chinese that Kim was steaming toward Beijing in his heavily armored train for his first foreign visit since assuming power. On Monday and Tuesday, senior officials in Washington admitted they had little idea whether the mysterious hunter green locomotive had indeed hauled Kim to China and not another senior North Korean official.

Senior administration officials spent Wednesday trying to decipher North Korean intentions following Kim's meeting with Xi. Some officials noted the optics -- including body language and rhetoric -- from both the North Koreans and the Chinese was hardly warm and fuzzy, determining the meeting appeared like it was for show.

China's ambassador in Washington, Cui Tiankai, traveled to the White House on Tuesday afternoon to brief officials, confirming that it was indeed Kim who had paid a visit to President Xi Jinping. In their conversations, they dictated a message to Trump from Xi which was subsequently shared with the President, who himself hailed the developments on Twitter.

"Received message last night from XI JINPING of China that his meeting with KIM JONG UN went very well and that KIM looks forward to his meeting with me," Trump wrote in the early morning hours of Wednesday. "In the meantime, and unfortunately, maximum sanctions and pressure must be maintained at all cost!"

Received message last night from XI JINPING of China that his meeting with KIM JONG UN went very well and that KIM looks forward to his meeting with me. In the meantime, and unfortunately, maximum sanctions and pressure must be maintained at all cost! — Donald J. Trump (@realDonaldTrump) March 28, 2018

That message provided at least some solace to US allies, some of whom are concerned Trump's eagerness to meet with Kim could lead to a loosening of sanctions. In a statement, the White House said it regarded Kim's visit as "further evidence that our campaign of maximum pressure is creating the appropriate atmosphere for dialogue with North Korea."

Japanese role

The document the Japanese are preparing ahead of Abe's talks with Trump will underscore the importance of maintaining sanctions pressure on North Korea, the person familiar with the efforts said. Tokyo hopes to warn Trump of ploys the North Koreans have attempted in the past and "traps" they may seek to lay, the person said.

Abe and other Japanese officials have long been the most skeptical players in the region about the possibility of direct talks between Trump and Kim. They have warned that North Korea could simply be attempting to buy time as it continues to develop its nuclear and missile programs.

Japan is also concerned the US could reach a deal with North Korea to restrict its long-range missiles, while allowing the North to retain its short and medium-range missiles that would still pose a threat to Japan.

Among world leaders, Abe has made perhaps the strongest efforts to appeal to Trump, visiting him in New York shortly after his election and gifting him with a golden golf club. At an exclusive country club outside Tokyo in November, the two men shared hamburgers and signed white ball caps that read: "Donald & Shinzo: Make Alliance Even Greater."

His anticipated visit to Florida next month will be his second time at Mar-a-Lago. His first sojourn to the oceanfront estate provided Trump a close-range view of Japan's concerns over North Korea. When Pyongyang launched an intermediate-range ballistic missile during dinner, the club's patio was transformed into an open-air situation room as the two leaders discussed how to respond.