In the meantime, the 25 million inhabitants of the South Korean capital, Seoul would be in grave danger from the enemy to the north and that U.S. and allied forces would be "subject to attack by conventional or chemical weapons" which would further delay their entry into the war.
He said it would be a "good thing" for North Korea and the world if he and Kim could become friends, saying it "might be a unusual thing to happen but it's certainly a possibility".
Speaking to reporters on Air Force One while making his way to Hanoi on Saturday, Mr Trump said he believed the Russian leader was being honest when he denied meddling.
The spectre of Pyongyang's nuclear weapons programme has loomed large over the President's trip, with one of the aims of tour to persuade China to put more pressure on the North.
That makes sense, but when world leaders such as former businessman Donald Trump and dictator Kim Jong-un are throwing school yard insults at each other, it's hard not to stick your nose in.
President Trump said Saturday that he would never call North Korean leader Kim Jong Un "short and fat", after Kim's government slammed Trump as an "old lunatic" following a recent speech in neighboring South Korea.
On Saturday, Kim's government responded by accusing Trump of trying to demonize North Korea, keep it apart from the global community and undermine its government.
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Shortly after that speech, North Korean officials told CNN, "We don't care about what that mad dog may utter", referring to Trump.
"That might be a odd thing to happen but it's a possibility", he said.
For the last months, the two leaders have been on two ends of a bitter war of words prompted in part by Pyongyang refusal to cease ballistic missile tests and the development of nuclear capabilities.
Those comments drew sharp criticism in Washington, where Rep. Adam Schiff (D-Calif.), the ranking member of the House Permanent Select Committee on Intelligence, said the president understands the truth about Russian interference, and is simply choosing to accept Putin's denials "over the solid evidence of our own intelligence agencies".
This isn't the first time Trump and Kim have traded barbs.
"And this artificial Democratic hit job gets in the way", Mr Trump said.