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 USA and allied forces would be "subject to attack by conventional or chemical weapons" which would further delay their entry into the war.
Asked at news conference with Vietnam's president later Sunday if he could become friends with the North Korean leader, Trump replied 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.
It comes on the same day Rodong Sinmun, the regime-run newspaper, warned the North would "surely win in the showdown" with the US and South Korea.
On Aug. 8, Trump threatened the regime with "fire and fury like the world has never seen", leading Kim to say he would consider sending missiles into the waters off the coast of Guam in "mid-August".
Trump's tweet was part of a series of tweets that included China's plan of "upping the sanctions" against North Korea and how Chinese President Xi Jinping wants to "denuclearize" the country.More news: Trump's ex-aide probed over plot to 'kidnap' Turkish cleric
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North Korean officials described Trump's trip as "nothing but a business trip by a warmonger to enrich the monopolies of the U.S. defense industry".
North Korea has dominated Mr Trump's Asia trip so far to Japan, South Korea, China and Vietnam.
That test would be the 15th ballistic missile test since Trump took office, and the first since September.
Town told ABC News that if Pyongyang conducted a test during or following Trump's trip, it would likely be with a Hwasong-14, what the US refers to as the KN-20 - a two-stage intercontinental ballistic missile (ICBM).
The president appeared to be responding to a statement from North Korea's foreign ministry Saturday, which warned that, "Reckless remarks by an old lunatic like Trump will never scare us or stop our advance".
Lieutenant General Jan-Marc Jouas said the U.S. could be swarmed by North Korean troops in a number of shocking documents leaked to Congress.