In a tweet this afternoon, the President said that military solutions were in place "should North Korea act unwisely".
The president said that the US military is ready in a tweet and said he hopes North Korea's leader, Kim Jong Un, "will find another path".
North Korea responded hours later by accusing the US of "war hysteria" and threatening to strike Guam.
Tensions between the two countries have escalated these weeks after North Korea tested two inter-continental ballistic missiles (ICBMs) in July.
Should North Korea attack the USA or one of its allies, Trump said that "things will happen to them like they never thought possible, OK?"
But the president's rhetoric appeared to have little effect, with North Korean state media quoting Kim as calling Trump "bereft of reason" and talking a "load of nonsense".
The tweet is a signal to Pyongyang that it needs to stand down and start the process of giving up its nuclear arms lest it persists on a path on which Secretary of Defense James Mattis promised would destroy North Korea. He did not comment on whether the U.S.is considering a pre-emptive strike on North Korea.
"He has disrespected our country greatly", Trump said of Kim Thursday.
He also warned on Thursday that, "if [Kim Jong Un] does something in Guam, it will be an event the likes of which nobody's seen before, what will happen in North Korea".
In the same speech, Trump said he'd favor talks with the hostile nation and more action on the diplomatic front than his White House predecessors.
Asked if the United States was ready if North Korea made a hostile act, he said: "We are ready".