WASHINGTON (BLOOMBERG) - President Donald Trump called the collapse of a planned summit with Pyongyang leader Kim Jong Un a setback for both North Korea and the world, and said the United States military is ready if necessary in the event of a conflict on the Korean peninsula.

"While many things can happen and a great opportunity lies ahead, potentially, I believe this is a tremendous setback for North Korea and indeed a setback for the world," Mr Trump said at the White House on Thursday (May 24), hours after releasing a letter to Mr Kim cancelling the meeting.

Mr Trump said he had spoken with Defence Secretary Jim Mattis and the leaders of South Korea and Japan. The US military is "ready if necessary", he said, and the two Asian allies "are not only ready should foolish or reckless acts be taken by North Korea, but they are willing to shoulder much of the financial cost or burden" of a conflict.

But Mr Trump also held out hope that the June 12 summit in Singapore could get back on track, or that he and Mr Kim could meet in the future.

"Nobody should be anxious. We have to get it right," he said.

Mr Trump sounded a positive note as he left the room, telling reporters "the dialogue was good until recently" with Mr Kim. And "Kim Jong Un wants to do what's right", adding, "I really believe that".

"It's only recently that this has been taking place and I think I understand why it's been taking place," he said. He declined to explain further, but Mr Trump said earlier this week that planning for the summit had been proceeding well until Mr Kim met on May 8 with his closest ally, Chinese President Xi Jingping, who is negotiating a trade dispute with Mr Trump.

In his letter to Mr Kim on Thursday pulling out of the summit, Mr Trump cited "tremendous anger and open hostility" in recent statements from Pyongyang.

North Korea hardened its rhetoric towards the US earlier on the same day, lashing out after remarks by Vice President Mike Pence and the White House national security adviser, Mr John Bolton, that had linked the country with Libya.

Ms Choe Son Hui, vice-minister of foreign affairs, called Mr Pence a "political dummy" and his comments "unbridled and impudent", according to an English-language statement from North Korea's state-run KCNA.

Ms Choe warned her nation was prepared for a "nuclear-to-nuclear" showdown if the US did not follow through on the summit.

"We can also make the US taste an appalling tragedy it has neither experienced nor even imagined up to now," she said, warning that she would recommend Mr Kim cancel the summit if US officials did not curb their language.

Mr Trump beat Mr Kim to it, issuing his own threat.

"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," Mr Trump wrote.