President Trump has cancelled the upcoming summit between the United States and the North Koreans in a new letter released on Thursday. Mr. Trump writes in the letter that he was "very much looking forward to being there with you. 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."

He also told Kim in the letter, "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."

Trump letter to Kim Jong Un canceling June summit.

According to South Korean news agency Yonhap, South Korea is still trying to figure out what's behind Mr. Trump's cancellation of the summit.

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Senate Foreign Relations Chairman Bob Corker told reporters, "I think the facts are that they've had difficulties communicating with them, there's a sense maybe that Kim Jong Un is not quite ready."

His comments appear to refer to insults from the North Koreans directed at Vice President Pence. North Korean Vice-Minister of Foreign Affairs Choe Son Hui, quoted by the state-run news agency, said of Pence Thursday, "I cannot suppress my surprise at such ignorant and stupid remarks gushing out from the mouth of the US vice-president."

Pence, in his own interview on Fox News on Wednesday, had seemed to threaten the North. "You know, as the president made clear, this will only end like the Libyan model ended if Kim Jong-un doesn't make a deal."

Under the Libyan model, Libyan leader Muammar Gaddafi dismantled his nuclear program in exchange for sanctions relief. The agreement was initially seen as a success for denuclearization, with Libya welcoming "If international inspectors to verify the dismantlement. But eight years later, Gaddafi was overthrown in a gruesome killing by Western-backed rebels.

Just Thursday, North Korea announced it had destroyed its Punggye-ri nuclear test site, and CBS News correspondent Ben Tracy was the only U.S. broadcast network correspondent on hand to witness several large explosions at Punggye-ri. He was among about two dozen international journalists who had been brought to the site to witness what the North said was the decommissioning of the nuclear test site.

Mr. Trump also wrote to Kim he felt a "wonderful dialogue was building up between you and me," and continued, "Some day, I look very much forward to meeting you." He thanked Kim for the recent release of three American hostages, calling it a "beautiful gesture and was very much appreciated."

"If you change your mind having to do with this most important summit," he adds, "please do not hesitate to call me or write."

The news comes after Mr. Trump has expressed some doubt over the June 12 date in Singapore, recently telling Fox News in an interview recorded Wednesday, "We'll see what happens."

Members of the administration including Secretary of State Mike Pompeo were still pushing for the June 12th date as recently as Wednesday. Pompeo testified before the House Foreign Affairs Committee where he said that the summit was "still scheduled for June 12." Pompeo spoke highly of his previous meetings with Kim, saying the U.S had made "zero concessions" to the leader, and discussed economic sanction assistance in return for complete and verifiable denuclearization.

He added, however, "We are clear-eyed about the regime's history. It's time to solve this once and for all. A bad deal is not an option. The American people are counting on us to get this right. If the right deal is not on the table, we will respectfully walk away."

On Thursday, Pompeo told lawmakers before the Senate Intelligence Committee that he "regrets the statements that North Korea have made over the past few days and the fact that we have not been able to conduct the preparation between our two teams that would be necessary to have a chance for a successful summit."

This is a developing story.