WASHINGTON - US President Donald Trump on Thursday called off his planned June summit with Kim Jong Un, blaming “tremendous anger” and “hostility” from the North Korean regime and warning Pyongyang against committing any “foolish or reckless acts.”

In a letter to Kim, Trump announced he would not go ahead with the high-stakes meeting set for June 12 in Singapore, and would instead pursue Washington’s “maximum pressure campaign” through sanctions on Kim’s regime.

Just before Trump announced the cancellation of the talks, North Korea declared it had “completely” dismantled its nuclear test site, in a carefully choreographed move portrayed by the isolated regime as a goodwill gesture ahead of the summit.

But the chances of success for the talks, which would have been an unprecedented face-to-face between a US and North Korean leader that Washington hoped would result in full denuclearization of the North, had recently been thrown into doubt on both sides.

Trump’s announcement - which prompted South Korea’s president to convene crisis talks of his top security advisors - came one day after Pyongyang hardened its rhetoric by attacking Vice President Mike Pence as “ignorant and stupid.”

“Sadly, based on the tremendous anger and open hostility displayed in your most recent statement, I feel it is inappropriate, at this time, to have this long-planned meeting,” read Trump’s letter to Kim, released by the White House. “The world, and North Korea in particular, has lost a great opportunity for lasting peace and great prosperity and wealth. This missed opportunity is a truly sad moment in history,” he said.

The US leader brandished the threat of America’s nuclear might in his letter, writing: “You talk about your nuclear capabilities, but ours are so massive and powerful that I pray to God they will never have to be used.”

And in later comments from the White House, Trump warned that South Korea and Japan stood ready to respond, along with the United States, “should foolish or reckless acts be taken by North Korea.”

“In the meantime, our very strong sanctions, by far the strongest sanctions ever imposed, and maximum pressure campaign will continue,” Trump added.

The Republican president held out hope that a meeting with Kim was still possible, stressing he had been looking forward to the event - talks which led him to openly entertain the notion it could earn him a Nobel Peace Prize. “It’s possible the existing summit could take place, or a summit at some later date. Nobody should be anxious. We have to get it right,” Trump said. “If and when Kim Jong Un chooses to engage in constructive dialogue and actions, I am waiting.”

Politically, Trump had invested heavily in the success of the planned summit. Privately, most US officials, as well as outside observers, believed it would go ahead despite the recent uptick in tensions.

The Pentagon is ready to respond to any “provocative actions” from North Korea, an official said Thursday after President Donald Trump canceled a summit with North Korean leader Kim Jong Un.

“We are in a boxer stance, we are ready to respond,” Lieutenant General Kenneth McKenzie, director of the US military’s Joint Staff, told reporters.

North Korea said it had “completely” dismantled its nuclear test site Thursday in a carefully choreographed move portrayed by the isolated regime as a goodwill gesture ahead of a potential summit with US President Donald Trump.

Invited foreign journalists at the scene described a series of explosions throughout the day, three of them in entry tunnels to the underground facility, followed by blasts that demolished a nearby barracks and other structures at the Punggye-ri test site in the country’s northeast. “There was a huge explosion, you could feel it. Dust came at you, the heat came at you. It was extremely loud,” Tom Cheshire, a journalist for Sky News who was among those invited to attend the ceremony, wrote on the British broadcaster’s website.

UN Secretary-General Antonio Guterres urged the parties to keep talking despite the summit cancellation, as did host Singapore.

Guterres said he was “deeply concerned” after US President Donald Trump cancelled a widely anticipated nuclear summit next month with North Korean leader Kim Jong Un.

In Seoul, South Korean President Moon Jae-in expressed his “deep regret” and summoned his top security aides to the presidential Blue House for emergency talks, his press secretary Yoon Young-chan said in a message to reporters, according to the country’s Yonhap news agency.

Officials in Seoul “are trying to figure out what President Trump’s intention is and the exact meaning of it,” Yonhap reported, quoting another presidential spokesman.