WASHINGTON — US President Donald Trump on Thursday called off a historic summit with North Korean leader Kim Jong Un scheduled for next month, citing Pyongyang's "open hostility," and warned that the US military was ready in the event of any reckless acts by North Korea.

Trump wrote a letter to Kim to announce his abrupt withdrawal from what would have been a first-ever meeting between a serving US president and a North Korean leader in Singapore on June 12.

"Sadly, based on the tremendous anger and open hostility displayed in your most recent statement, I feel it would be inappropriate, at this time, to have this long-planned meeting," Trump wrote. "Please let this letter serve to represent that the Singapore summit, for the good of both parties, but to the detriment of the world, will not take place."

Earlier on Thursday, North Korea had repeated its threat to pull out of the summit, which was intended to address concerns about its nuclear weapons program, and warned it was prepared for a nuclear showdown with Washington if necessary.

A White House official said a North Korean official's condemnation of US Vice President Mike Pence as a "political dummy" was "the last straw" that led to canceling the summit.

A second White House official said a major factor in Trump walking away was the possibility of a nuclear conflict raised by a North Korean official if diplomacy failed.

"The North Koreans literally threatened nuclear war in the statement released last night," she said. "No summit could be successful under these circumstances."

In a statement at the White House, Trump said he remained open to dialogue but had spoken to Defense Secretary Jim Mattis and warned North Korea against any "reckless act."

"We are more ready than we have ever been before," Trump said. He said US allies South Korea and Japan also were ready to shoulder much of the financial burden "if an unfortunate situation is forced upon us" by North Korea.

"While many things can happen and a great opportunity lies ahead potentially, I believe that this is a tremendous setback for North Korea and indeed a setback for the world.”

Asked if cancelation of the summit increased the risk of war, Trump replied: "We'll see what happens."

US stocks were down in afternoon trading but were well off the session lows hit after Trump canceled the summit and threatened to impose tariffs on auto imports.

Trump said the United States would continue its "maximum pressure" campaign of sanctions to press North Korea to give up its nuclear weapons.

"North Korea has opportunity to end decades of poverty and oppression by following the path of denuclearization, and joining the community of nations," he said.

Last month Trump had praised the authoritarian Kim as "very honorable" while preparing for the summit but the outlook for the meeting suffered a setback this month after North Korea angrily rejected the notion that it would agree to unilateral nuclear disarmament as the United States has demanded.

Trump canceled the summit just a few hours after North Korea followed through on a pledge to blow up tunnels at its main nuclear test site, which Pyongyang said was proof of its commitment to end nuclear testing.

A small group of international media selected by North Korea witnessed the demolition of tunnels at the Punggye-ri site on Thursday.

The apparent destruction of what North Korea said was its only nuclear test site had been widely welcomed as a positive, if largely symbolic, step. Kim has declared his nuclear force complete, amid speculation the site was obsolete anyway.

The Pentagon said it was too early to give an assessment of the action at Punggye-ri but the site could be put back into service or re-established elsewhere.

South Korean President Moon Jae-in, whose government had helped set up the summit, said he was "perplexed" by the cancelation and urged Trump and Kim to talk directly to each other.

Moon had met with Trump at the White House on Tuesday and urged him not to let a rare opportunity for a meeting with reclusive North Korea slip away.

The reference to Pence that offended the White House came in a statement released by North Korean media and citing Vice Foreign Minister Choe Son Hui. She had called Pence a "political dummy" for comparing North Korea — a "nuclear weapons state" — to Libya, where Muammar Gaddafi gave up his unfinished nuclear development program, only to be later killed by NATO-backed fighters.

"Whether the US will meet us at a meeting room or encounter us at nuclear-to-nuclear showdown is entirely dependent upon the decision and behavior of the United States," Choe said.

US national security adviser John Bolton first advocated a Libya as a model of North Korea's disarmament. That incensed North Korea, which said the reason it had its nuclear arms was to ensure it did not end up like Libya and Gaddafi.

Trump had raised expectations for a successful summit after North Korea released three Americans this month, which Trump in his latter called "a beautiful gesture" by Kim.

While Trump's letter left the door open for talks with Kim, chances for a quick rescheduling appear remote and cancelation of the meeting will renew fears of a return to conflict on the Korean peninsula.

Trump's letter also referred to the possibility of war. "You talk about your nuclear capabilities, but ours are so massive and powerful that I pray to God that they will never have to be used," he said.

North Korea's pursuit of nuclear weapons has been a source of tension on the Korean peninsula for decades, as well as antagonism with Washington, but escalated into fears of war last year after North Korea said it had tested an H-bomb and developed a missile capable of hitting the United States.

The rhetoric reached new heights under Trump as he mocked Kim as "little rocket man" and in address at the United Nations threatened to "totally destroy" North Korea if necessary. Kim had called Trump mentally deranged and threatened to "tame" him with fire.

US Secretary of State Mike Pompeo, who traveled to North Korea twice to prepare the summit, meeting Kim both times, said Pyongyang had not responded in recent days to queries about the meeting.

Cancelation of the summit denies Trump what supporters hoped could have been the biggest diplomatic achievement of his presidency, and one worthy of a Nobel Peace Prize.

It comes at a time when Trump’s withdrawal from the Iran nuclear deal has drawn criticism internationally, his moving of the US embassy to Jerusalem has fueled violence on the Israel-Gaza border and he is on the defensive over an investigation into alleged Russian meddling in the 2016 election.

Senator Bob Menendez, the top Democrat on the Foreign Relations Committee, said he had not had a sense that the administration had engaged in the detailed preparations necessary for a successful summit with Kim. He also suggested that rhetoric from top administration officials might not have been appropriate ahead of the meeting.

Robert Einhorn, a non-proliferation expert at the Brookings Institution, said it seemed Trump had realized he was not going to be able to get an assurance from Kim of North Korea’s willingness to give up its nuclear weapons.

"He was, I think, reluctant to go to Singapore and come up short," he said. "This probably was the best choice he could make — much better than having a meeting that would deepen the divisions, lead to angry recriminations and set back any prospect for getting back on track." — Reuters