President Trump’s abrupt decision Thursday to abort a summit next month with North Korean leader Kim Jong Un left the White House scrambling to explain the outcome to allies amid fears that the collapse of talks would mean a return to heightened tensions between nuclear powers in East Asia.

Trump announced he was pulling out of the planned meeting in Singapore on June 12 in a letter to Kim that came less than 12 hours after a North Korean official had personally disparaged Vice President Pence and warned of a nuclear showdown if the United States did not alter its tone ahead of the summit.

In a missive that aides said the president dictated, Trump was by turns regretful of the missed opportunity and adamant that he would not tolerate the “tremendous anger and open hostility” from North Korea. The president pointedly warned Kim that he oversees a nuclear weapons arsenal that is “so massive and powerful that I pray to God they will never have to be used.”

Trump’s aim was to remind the young dictator “of the real balance of power,” a senior White House official told reporters. “The president’s overarching goal isn’t a meeting. It’s always been the denuclearization of the Korean Peninsula. He will never compromise the safety and security of the United States and its allies.”

In remarks at the White House, Trump left the door open for the summit to be rescheduled and a North Korean official, perhaps eager to foist blame on Trump, responded that Pyongyang remains ready to meet “at any time.”

A summit is urgently needed to deal with the “grave hostilities” in the relationship, vice minister Kim Gye Gwan said, according to North Korea’s state media. “Leader Kim Jong Un had focused every effort on his meeting with President Trump.”

But senior White House aides emphasized that a rescheduled meeting was unlikely anytime soon, citing a series of “broken promises” from Pyongyang that have frayed trust.

Among other things, they said, North Korea failed to show up for a key logistical planning meeting with a U.S. delegation in Singapore last month. Pyongyang offered no communications with Washington for a week before the bellicose statement late Wednesday, White House aides said.

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

The breakdown represented a stunning turn of events for Trump’s boldest and riskiest foreign policy endeavor, coming just 10 weeks after the president impulsively agreed in March to meet Kim after a year of escalating hostilities that had raised the specter of a military confrontation on the Korean Peninsula.

Amid visions of a historic peace deal that had eluded his predecessors and inspired talk of a Nobel Peace Prize among his core supporters, Trump rushed headlong into the summit process — disregarding warnings from his own aides that North Korea has long been an untrustworthy negotiating partner and that Kim’s intentions remained unclear.

On Thursday, the president’s cancellation of talks sent renewed shock waves through East Asia, where the other major powers — South Korea, Japan and China — had jockeyed for leverage over the prospects that warming relations could reshape alliances.

Trump and his aides have blamed Beijing for influencing Kim in recent weeks to take a harder line, souring relations ahead of the summit. But the president’s decision was likely felt most acutely in Seoul, where President Moon Jae-in, fearful of the escalating threats between Washington and Pyongyang last year, had staked his presidency on a policy of engagement with the North and positioned himself as the intermediary between Trump and Kim.

Moon, who met with Kim last month in the Korean demilitarized zone, had visited Trump at the White House on Tuesday in a desperate bid to save the Singapore summit. Instead, having arrived back in Seoul just hours earlier, he appeared blindsided by the news, convening an emergency midnight meeting with aides at the presidential Blue House.

Trump’s decision came on the same day that North Korea announced that it had followed through on a pledge to destroy its underground nuclear testing facilities, although White House officials said the action could not be verified because Pyongyang did not admit international nuclear security experts to the site.

Moon said he was “very perplexed and sorry” that the summit had been canceled, adding that “the denuclearization of the Korean Peninsula and ensuring a permanent peace are historic tasks that cannot be delayed or forsaken.”

White House aides sought to tamp down fears that the collapse of the talks would leave the president without a strategy to deter North Korea from further destabilizing the region. The aides emphasized that the administration would maintain its policy of “maximum pressure” through economic sanctions and diplomatic isolation of Pyongyang.

Secretary of State Mike Pompeo, testifying before a Senate panel Thursday, said the United States would continue to work with allies to increase the sanctions, including intercepting ship-to-ship transfers and shipments of refined petroleum to North Korea.

“The pressure campaign continues,” Pompeo said. “That won’t change.”

But critics said Trump’s hasty jump into a poorly thought-out summit process had left the United States in a weakened position. Kim’s outreach to Seoul and Beijing, where he visited twice with President Xi Jinping, has fractured the pressure campaign, analysts said, and Trump’s personal dalliance with Kim elevated the stature of a brutal, authoritarian regime on the global stage.

“The Administration’s approach to North Korea from start to finish has been one long amateur hour,” Susan E. Rice, who served as national security adviser under President Barack Obama, wrote on Twitter. “In that way, it is not much different from Trump’s approach to many other issues. But the difference is that with North Korea the stakes could not be higher.”

Victor Cha, who served as a top Asia policy aide in the George W. Bush administration, noted that Trump had plenty of warnings that the North Koreans were untrustworthy partners given that the regime had violated past nuclear agreements with the United States.

“This is what happens when you jump too early to a summit,” Cha said. “If this breakdown means North Korea is no longer beholden to their missile-testing moratorium, that takes us to a very bad place.”

The chief concern, analysts said, was a return to the threats and hostilities between the countries that marked Trump’s first year in office, when the administration was said to be exploring options that included potential military actions.

Though Kim had attempted to offer signs of goodwill, including the release of three American prisoners this month, top Trump aides, including national security adviser John Bolton, have remained skeptical of talks with Pyongyang.

Bolton’s suggestion, echoed by Pence, that North Korea must relinquish its nuclear program completely before receiving reciprocal benefits from the United States — a situation he compared to Libya’s actions in 2003 — inspired fierce denunciations from Kim’s aides over the past two weeks.

During his remarks, Trump said he had consulted with Defense Secretary Jim Mattis, and he praised the U.S. military as “by far the most powerful anywhere in the world.”

“Hopefully, positive things will be taking place with respect to the future of North Korea,” the president added. “But if they don’t, we are more ready than we have ever been before.”

Fifield reported from Tokyo. John Hudson, Josh Dawsey, Anne Gearan and Seung Min Kim in Washington contributed to this report.