U.S. President Donald Trump on Thursday canceled his planned June meeting with North Korean leader Kim Jong Un, calling it a "tremendous setback" for the North and the world while warning that the U.S. military is ready to respond to any "foolish or reckless acts" by Pyongyang.

In a letter to Kim, Trump cited "tremendous anger and open hostility" in a recent statement released by North Korea. "I feel it is inappropriate, at this time, to have this long-planned meeting," the U.S. president said, damping hopes for lasting peace in Northeast Asia and beyond.

The cancellation of what would have been the first-ever summit between the two countries is likely to rekindle tensions between Trump and Kim, who have been at odds over how to proceed with denuclearization on the Korean Peninsula.

[Getty/Kyodo]

It also came after North Korea claimed Thursday that it has "completely" dismantled its only known nuclear test site as a gesture, though not verified by outside experts, that the country is taking a measure toward denuclearization. Pyongyang freed three American detainees earlier this month.

The letter, a copy of which was released by the White House, referred to Pyongyang's threat of a nuclear war with Washington and its insulting of Vice President Mike Pence as a "political dummy" in a statement issued Thursday by Vice Foreign Minister Choe Son Hui.

Speaking about his decision to "terminate" the June 12 meeting with Kim in Singapore, Trump said Washington's "very strong sanctions" on Pyongyang and the U.S.-led "maximum pressure" campaign will continue in an effort to compel it to give up nuclear weapons and ballistic missiles.

"Our military -- which is by far the most powerful anywhere in the world and has been greatly enhanced recently, as you know -- is ready if necessary," Trump said in remarks at the White House, in reference to a possible resumption of nuclear and missile tests by the North or other provocations.

The president said he has spoken with South Korea and Japan about his decision, and that the key allies "are...ready should foolish or reckless acts be taken by North Korea."

Seoul and Tokyo are also "willing to shoulder much of the cost of any financial burden, any other costs associated by the United States in operations if such an unfortunate situation is forced upon us," he said.

But at the same time, Trump left the door open for dialogue, including the possibility that the summit could yet be rescheduled.

"North Korea has the opportunity to end decades of poverty and oppression by following the path of denuclearization and joining the community of nations," he said. "If and when Kim Jong Un chooses to engage in constructive dialogue and actions, I am waiting."

While thanking Kim for the release of the three Americans detained in North Korea, Trump wrote in the letter, "If you change your mind having to do with this most important summit, please do not hesitate to call me or write."

After extending diplomatic outreach since taking part in the Pyeongchang Winter Olympics in South Korea in February, North Korea abruptly changed its tone last week when it threatened to call off the Trump-Kim summit if Washington insisted on "unilateral nuclear abandonment" by Pyongyang.

Speaking at a Senate hearing, Secretary of State Mike Pompeo said North Korea had not responded "over the past many days" to U.S. requests to put the two sides' preparation teams together in preparation for the summit.

A senior White House official said separately that a North Korean delegation failed to show up for a meeting that had been scheduled last week in Singapore with U.S. officials to jointly work on logistical preparations for the summit.

"The North Koreans didn't tell us anything. They simply stood us up," the official said, speaking on condition of anonymity.

"The United States has over the past week made numerous attempts to communicate with the North Koreans, but they never responded," the official said.

The first communication Washington had received from Pyongyang in a week, the official said, was a statement by Choe released Thursday by the North's official media.

While issuing another threat to cancel the summit, Choe said, "Whether the U.S. 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."