South Korean President Moon Jae-in discussed North Korea’s recent behavior with President Donald Trump in a 20-minute phone conversation on Sunday. According to South Korea’s presidential Blue House, the two leaders reaffirmed their commitment to work together for a successful summit between the U.S. and North Korea in June.

Korea JoongAng Daily found the phone call unusual because Moon will arrive in Washington on Monday and is scheduled to meet with Trump in person on Tuesday:

Yoon Young-chan, Moon’s senior secretary for public affairs, said the two presidents agreed in their conversation to closely coordinate with each other through events like their upcoming meeting together to ensure the North-U.S. summit is successful. But the timing of the phone call, just days before Moon was set to meet with Trump in person, raised questions about why it was necessary. A senior presidential official speaking on the condition of anonymity told reporters there had been no change in the status quo and that Moon and Trump simply exchanged views on the current diplomatic environment involving North Korea. The official said the conversation mostly involved Trump asking questions and Moon answering, without elaborating further. “Given the call was made at late night Washington time, it appears to show strong determination from Trump for a successful North-U.S. summit,” the official said.

The White House press statement about the call said Trump and Moon discussed “recent developments in North Korea” and promised to “continue their close coordination ahead of President Trump’s June 12 meeting with North Korean leader Kim Jong Un.”

“President Trump said he looks forward to continuing their conversation when President Moon visits the White House on May 22,” the White House added.

The Associated Press speculated that not only is Moon worried about Trump’s pushing him out of the “diplomatic driver’s seat,” but he might be concerned that if North Korea backs out of the summit, his own reputation will be severely tarnished:

Pyongyang’s surprise move last week to break off a high-level meeting with Seoul over U.S.-South Korean military drills while threatening to cancel next month’s summit between North Korean leader Kim Jong Un and Trump cooled what had been an unusual flurry of diplomatic moves from the country after a provocative series of nuclear and missile tests. It also underscored Seoul’s delicate role as an intermediary between Washington and Pyongyang and raised questions about Moon’s claim that Kim has genuine intent to deal away his nukes. Seoul may lose much of its voice if Trump chooses to deal more directly with China, North Korea’s only major ally, which is refusing to be sidelined in the global diplomatic push to resolve the nuclear standoff.

Moon’s fears of Trump’s sidestepping South Korea to deal directly with China may have been somewhat allayed by Trump’s accusation on Monday morning that the Chinese are helping North Korea evade sanctions. Indeed, this comment from Trump might have been inspired by something Moon said during their telephone conversation:

China must continue to be strong & tight on the Border of North Korea until a deal is made. The word is that recently the Border has become much more porous and more has been filtering in. I want this to happen, and North Korea to be VERY successful, but only after signing! — Donald J. Trump (@realDonaldTrump) May 21, 2018

Alternatively, U.S. officials who spoke anonymously to the New York Times during the weekend said Trump is the one who grows increasingly concerned that his summit with Kim could become a “political embarrassment.”

According to these sources, Trump was “both surprised and angered by a statement issued on Wednesday by the North’s chief nuclear negotiator, who declared that the country would never trade away its nuclear weapons capability in exchange for economic aid.” The administration officials said Trump called Moon on Sunday to ask why these intransigent statements by North Korea are so much different than Moon’s private assurances that Kim Jong-un is sincerely interested in denuclearization.

The Washington Post described Trump as using the Sunday phone call to seek Moon’s “interpretation of Pyongyang’s shift to a harder-line position last week.” The Singapore summit is less than three weeks away, and U.S. advance teams are already working out the logistics, so every day counts.

The Post mentioned that National Security Adviser John Bolton “has been telling colleagues that he doesn’t trust that the summit will go well, and he has reiterated his long-standing distrust of the North Koreans,” so perhaps Trump was looking for an optimistic take to balance that pessimistic view.

A senior U.S. official told the Post that euphoria over Moon’s own summit meeting with Kim is overblown, pointing out that the North Koreans have already failed to live up to some of the commitments made by their leader.