"The DPRK regime's actions will continue to be grossly overmatched by ours and would lose any arms race or conflict it initiates", Mattis said.
Referring to the volatile Kim, Trump said, "He has been very threatening beyond a normal state, and as I said, they will be met with fire and fury, and frankly power the likes of which this world has never seen before". They evoked President Harry Truman's announcement of the United States atomic bomb dropped on Hiroshima, Japan, in 1945, in which he warned of "a rain of ruin from the air, the like of which has never been seen on this earth".
Media reports said the North has successfully miniaturised a nuclear warhead that can fit inside its missiles.
Mattis said the U.S.is committed to a diplomatic resolution to the conflict but "the combined allied militaries now possess the most precise, rehearsed and robust defensive and offensive capabilities on Earth".
Despite the statement by Mattis, U.S officials say that for now, additional assets have not been moved to the region and the threat assessment has not changed.
North Korea will continue to be grossly overmatched by the US and its allies, he said, "and would lose any arms race or conflict it initiates".
Guam hosts strategic USA military installations - including both a naval and an air force base - and is more than 3,400km from North Korea.
Meanwhile, France is urging all sides "to act responsibly and to de-escalate" tensions over North Korea's nuclear ambitions.
"There's obviously growing concern, and that concern is fueled when the president of the USA deliberately uses language that escalates sort of the war of words and I think, unfortunately, matches the tone and tenor of North Korean officials", Kildee said.
It cited USA intelligence officials. On Saturday, for example, members of the U.N. Security Council approved new economic sanctions on the regime.
Responding to Trump's comments, North Korea said it was considering a strike on Guam that would create "an enveloping fire". It was the longest, and farthest ballistic missile test in the history of North Korea, officials told Fox News at the time.
Tens of thousands of USA troops remain stationed in South Korea and in nearby Japan, the only country to have been attacked with nuclear weapons.