President Trump has said that North Korean leader Kim Jong Un will “truly regret” any action he takes against Guam.
He told reporters: “I hope that they are going to fully understand the gravity of what I said, and what I said is what I mean.
“Those words are very easy to understand.”
Mr Trump was speaking at his Bedminster golf resort in New Jersey, where he is on holiday.
Hours earlier the President tweeted that military solutions were “locked and loaded” should North Korea “act unwisely”.
Mr Trump dismissed critics as attacking his words only “because it’s me”, adding that his rhetoric would be welcomed as “a great statement” if “somebody else” had said it.
He said that millions of Americans supported him because “finally we have a President that’s sticking up for our nation and frankly sticking up for our friends and our allies”.
But German Chancellor Angela Merkel urged both sides to tone down their tough talk.
She said: “I don’t see a military solution and I don’t think it’s called for.
“I think escalating the rhetoric is the wrong answer.”
But Mr Trump responded to this by saying: “Let her speak for Germany. She’s certainly not referring to the United States.”
Russia’s foreign minister Sergei Lavrov said Moscow is deeply concerned about the developments, estimating that the risk of a military conflict is “very high”.
Mr Lavrov added that “when it comes close to a fight, the one who is stronger and wiser should be the first to step back from the brink”.
The latest words come after nearly a week of threats being exchanged between the US and North Korea, with the latter vowing to send missiles flying over western Japan to land near the US Pacific territory of Guam.
This came after Mr Trump had promised North Korea “fire and fury like the world has never seen” if it provoked the US.
Japan earlier began deploying land-based Patriot interceptors at four cities, while American and South Korean officials said they plan to go ahead with military exercises later this month, which North Korea has described as a rehearsal for war..