Mattis made the remarks to reporters at an event in California on Thursday, adding that the US prefers a diplomatic approach to the North Korean threat.
He warned that the tragedy of war is well known and a military conflict could be 'catastrophic'. "It doesn't need another characterization beyond the fact that it would be catastrophic".
It says North Korea will complete a plan by mid-August for the "historic enveloping fire at Guam", convey it to the commander in chief of its nuclear force and then "wait for his order".
The Pentagon chief on Wednesday issued a carefully worded statement telling North Korea it would be "grossly" outmatched in any conflict with the USA, and warning Pyongyang to stop considering any action that risked "the destruction of its people".
A separate statement by the Korean People's Army (KPA) also threatened to turn Seoul into "a sea of fire", comments condemned yesterday as "thoughtless" by South Korean military officials.
After days of fiery rhetoric from both the USA and North Korea, Mr Mattis said war would be "catastrophic" and that diplomacy was gaining results.
And while Mattis warned on Wednesday that North Korea would be "grossly overmatched" by the US and its allies if a military conflict were to break out, some senior administration officials have provided a counterpoint to the President's more inflammatory rhetoric.
And Secretary of State Rex Tillerson has made clear he is open to negotiations if Pyongyang freezes its nuclear and missile programs.
"[Kim Jong-un] has been very threatening beyond a normal statement, and as I said, they will be met with fire, fury, and, frankly, power, the likes of which this world has never seen before", Trump concluded.