U.S. President Donald Trump on Thursday called off a planned historic summit with North Korean leader Kim Jong Un, even after North Korea followed through on a pledge to blow up tunnels at its nuclear test site, Reuters reported.

Referring to a scheduled June 12 meeting with Kim in Singapore, Trump said in a letter to the North Korean leader: “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 called it “a missed opportunity” and said he still hoped to meet Kim someday.

The North Korean mission to the United Nations did not immediately respond to a request for comment on Trump’s cancellation of the summit.

U.S. stocks dropped sharply on the news, with the benchmark S&P 500 Index falling more than half a percent in about 10 minutes. Investors turned to U.S. Treasury debt as a safe alternative, driving the yield on the 10-year note, which moves inversely to its price, down to a 10-day low and back below the psychologically important 3 percent level. The U.S. dollar also weakened broadly, particularly against the Japanese yen, which climbed to a two-week high against the greenback.

“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,” Trump wrote.

“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.

Earlier on Thursday, North Korea repeated a threat to pull out of the summit with Trump next month and warned it was prepared for a nuclear showdown with Washington if necessary.