President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE on Thursday canceled his nuclear summit with North Korean leader Kim Jong Un, citing Kim’s “tremendous anger and open hostility” toward the United States.

“I feel it is inappropriate, at this time, to have this long-planned meeting,” Trump wrote in a letter to Kim that was released by the White House.

Speaking later at the White House, Trump said his “maximum pressure campaign” against North Korea would continue and threatened a military response if Kim resumes nuclear activity.

But he also left the door open for the talks to take place if Kim takes “constructive” steps toward peace.

"It's possible that the existing summit could take place, or a summit at some later date,” Trump said during a bill signing ceremony.

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The two were scheduled to meet June 12 in Singapore in what would have been the first meeting between a sitting U.S. president and a North Korean leader.

The sudden cancellation is a major setback for Trump, who made nuclear diplomacy with Pyongyang his top foreign policy goal. The president was eager to have the meeting, openly musing about how brokering an agreement with Kim could earn him a Nobel Peace Prize.

"I believe this is a tremendous setback for North Korea and, indeed, a setback for the world," Trump said.

Trump agreed to sit down with Kim in March, following months of hurling insults and threats at the North Korean leader.

But tensions between the U.S. and North Korea rapidly escalated over the past week as both sides wrangled for leverage ahead of the talks, developments that foreshadowed the cancellation.

Trump publicly expressed doubt about whether the summit would take place after North Korea threatened to pull out if the U.S. continued to demand full denuclearization.

On Wednesday, a top North Korean official launched another verbal fusillade at the Trump administration, calling Vice President Pence a “political dummy” and saying his government is just as ready to inflict an “appalling tragedy” on the U.S. as it is to talk.

The president returned the favor in his letter, writing that while Kim likes to “talk about” his nuclear capabilities, “ours are so massive and so powerful that I pray to God they will never have to be used.”

Trump, however, did not close the door to sitting down with the reclusive North Korean leader in the future.

“Some day, I look very much forward to meeting you,” he wrote. “If you change your mind having to do with this most important summit, please do not hesitate to call me or write.”

Despite the mounting tensions, Trump’s decision appeared sudden. U.S. officials were expected to meet over the weekend with North Korean representatives to plot logistics for the Singapore talks.

But Secretary of State Mike Pompeo Michael (Mike) Richard PompeoOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Overnight Defense: House Democrats unveil stopgap spending measure to GOP opposition | Bill includes .6B for new subs | Trump issues Iran sanctions after world shrugs at US action at UN Navalny calls on Russia to return clothes he was wearing when he fell ill MORE said Thursday that Pyongyang has been unresponsive to Washington’s efforts to prepare for the meeting.

“We have received no response to our inquiries from them,” he said during testimony before the Senate Foreign Relations Committee, adding “I don’t believe in that sense that we’re in a position to believe that there could be a successful outcome.”

Trump's decision to agree to talks with Kim was similarly unexpected. He immediately accepted North Korea's offer after it was presented by South Korean officials during a March 8 meeting at the White House, even before consulting with some members of his national security team.

Trump’s announcement on Thursday appeared to catch South Korea by surprise. President Moon Jae-in urged Trump and Kim to recommit to direct talks after convening an emergency meeting with his national-security team.

"Denuclearization of the Korean Peninsula and the establishment of permanent peace are historic tasks that can neither be abandoned nor delayed," Moon said in the meeting, according to South Korea’s Yonhap News Agency.

Trump said he informed South Korea and Japan of his decision, but it is unclear if that happened before it was made public.

The last couple months of diplomacy with North Korea did produce some wins for the U.S.

Just hours before Trump’s announcement, Kim made good on his promise to demolish a nuclear test site, a goodwill move ahead of the talks. The U.S. also secured another concession from North Korea earlier this month when it released three American detainees.

But some foreign policy experts had warned that striking a nuclear deal with Kim Jong Un would be next to impossible.

"The summit was bound to fail as @realDonaldTrump admin badly overestimated what NK would agree to; the issue was/is US willingness to accept an outcome short of total denuclearization," tweeted Council on Foreign Relations President Richard Haas. "All or nothing foreign policy w NK, Iran, China trade risks producing nothing or conflict."

Harry Kazianis, director of defense studies at the Center for the National Interest, said the United States and North Korea were simply too far apart on what they wanted from the summit.



Pyongyang, he said, wanted to follow the “classic Kim regime model” of setting an aspirational goal in exchange for economic relief — something Trump would never agree to.



“It was night and day. There was no way to bridge that within weeks,” Kazianis said.



As for what comes next, he said, the next two to three days will be key in seeing whether talks can quietly continue through back channels or if tensions ratchet up to where they were last year.



“If they lob a rhetorical bomb overnight, that’s a sign that we’re going back to the brink and North Korea is going to start testing missiles again,” he said.

Robert Gallucci, the chief U.S. negotiator during the 1994 North Korean nuclear crisis, said he does not read Trump’s letter as a negotiating tactic to get Kim to give into his demands at any rescheduled summit. Rather, he said, it appears the Trump administration was coming to the realization that it “did not to like smell of all of this.”



And while Pyongyang’s recent statements may have been more about posturing than actually cancelling the summit, Gallucci said, it is likely not upset at the outcome.



“I don’t think they were afraid of that happening or they wouldn’t have said what they said about the vice president,” he said.



He added that he thinks it would be “implausible” for back channel talks to happen now.



“The world is not safer the day after the decision is made not to meet,” he said. “I did not enjoy 2017 as a North Korea-U.S. year, and I’m afraid we’re back in the soup now.”

Rebecca Kheel contributed.

Updated at 12:41 p.m.