SEOUL, South Korea (AP) — Just a month ago, South Korean President Moon Jae-in was smiling with pride as he grasped the hands of North Korean ruler Kim Jong Un, a young and brash leader with a nuclear arsenal, as they announced aspirations of a nuclear-free peninsula and permanent peace.

South Koreans pinned their hopes on a summit to come between Kim and President Donald Trump. Their euphoria shifted to unease before sinking into a funk upon learning as they woke Friday that Trump had canceled the summit, citing hostility in recent North Korean comments.

His decision underscores the delicacy of Seoul's intermediary status but also the credibility problems Moon faces over his claim that Kim is genuinely interested in negotiating away his nuclear weapons.

Moon had meticulously brokered the summit between Trump and Kim and claimed the "driver's seat" in trying to resolve the nuclear standoff, but he said Trump's cancellation left him "very perplexed."

Critics, though, say Moon has been an overeager matchmaker rather than an honest broker, driving up unrealistic expectations in both Washington and Pyongyang for a quick breakthrough when they remain far apart on the future of Kim's nuclear program.

Choi Kang, vice president of Seoul's Asan Institute for Policy Studies, said Trump's decision has put Moon in a difficult quandary. It will be hard for Moon to continue engagement with Pyongyang at the cost of widening a rift with Washington. Moon will also have to deal with a public split deep along ideological lines, with progressives expressing anger over Trump and conservatives criticizing Moon for being too soft on Pyongyang.

"It's a big dilemma and I can't think of any clear way out of it," Choi said. "Inter-Korean relations have become a substructure of U.S-North Korea relations."

South Korea was clearly blindsided by Trump's decision to back away from the summit, which was announced in a letter to Kim. It came just hours after the South Korean government issued a statement welcoming the North's dismantling of its nuclear test site, calling it a "meaningful first step" toward denuclearization. It came just days after Trump hosted a White House meeting with Moon where they expressed optimism for successful talks in Singapore.

In his letter, Trump objected specifically to a statement from senior North Korean diplomat Choe Son Hui, who referred to Vice President Mike Pence as a "political dummy" for his earlier comments on the North and said it was up to the Americans whether they would "meet us at a meeting room or encounter us at nuclear-to-nuclear showdown."

Trump didn't entirely kill the summit, writing to Kim "please do not hesitate to call me or write."

But Trump also threw Moon under the metaphorical bus. In an apparent response to Choe's claim that Washington had asked for dialogue first, Trump wrote: "We were informed that the meeting was requested by North Korea, but that to us is totally irrelevant."

South Korea maintained an optimistic tone on Friday, referring to the North's conciliatory reaction to Trump's announcement and its hope the summit could take place later.

"There have been no changes in the stance of related parties in that the problem should be solved through dialogue," said Baek Tae-hyun, spokesman of Seoul's Unification Ministry.

But clearly Moon faces the greatest test of his diplomatic skills since he used his hosting role for the Winter Olympics in February to renew the push to resolve the nuclear standoff with Pyongyang.

North Korea sent hundreds of athletes, performers and dignitaries to the Pyeongchang Games, including Kim's sister who conveyed her brother's desire for a summit with Moon. Moon later brokered a summit between Kim and Trump.

The chill on the inter-Korean warmth set in when North Korea canceled a high-level meeting with the South over U.S.-South Korean military drills. The North also lashed out at Trump's hard-line national security adviser, John Bolton, before it criticized Pence and said it has no interest in talks that amount to a unilateral surrender of its nukes.

The North's statements raised further doubts about Moon's claim that Kim can be persuaded to abandon his nuclear facilities, materials and weapons in a verifiable and irreversible way in return for credible security and economic guarantees.

Comments in North Korea's state media indicate Kim saw any meeting with Trump as an arms control negotiation between legitimate nuclear states, rather than a process to surrender his nukes.

For decades, North Korea has been pushing a concept of "denuclearization" that bears no resemblance to the American definition, vowing to pursue nuclear development unless Washington removes its troops from South Korea and the nuclear umbrella defending South Korea and Japan.

Moon and Kim's vague pledge for the "complete denuclearization" of their peninsula after last month's inter-Korean summit has done little to quiet questions over Kim's true intent.

South Korea may lose much of its voice if Trump chooses to deal more directly with Kim or seeks a broader resolution including China, North Korea's only major ally. Moon may lose the driver's seat for good if Pyongyang reverts to provocative weapons tests, which would bring back U.S. talk of using military force against the North.

"It's an awkward and tough situation but the talks haven't been broken yet," said Koh Yu-hwan, a North Korea expert at Seoul's Dongguk University and an policy adviser to Moon. "South Korea has to be more active as a mediator to reduce the distrust between both sides."

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Associated Press corresponded Hyung-jin Kim contributed to this report.