The State Department says President Donald Trump is "on the same page" with the rest of US government with his fiery threat to North Korea.
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. The tragedy of war is well enough known.
"The American effort is diplomatically led, it has diplomatic traction, it is gaining diplomatic results and I want to stay right there right now", he added.
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 North Korean statement also says the military action its army "is about to take" will be effective for restraining America's "frantic moves" in and near the southern part of the Korean Peninsula.
President Trump, however, drew criticism earlier this week when he threatened North Korea with "fire and fury."Mattis told reporters on the flight Wednesday the president has the prerogative to use whatever rhetoric he chooses".
The South Korean government has maintained that it is open to dialogue with the North, and that a diplomatic solution to the DPRK issue could be achieved.
Tillerson's more moderate line, however, was seemingly dismissed by Deputy Assistant to the President Sebastian Gorka in an interview with BBC on Thursday.
Trump responded to a question about a new report in which US intelligence analysts have "assessed that North Korea has produced a miniaturized nuclear warhead, according to multiple sources familiar with the analysis of North Korea's missile and nuclear program", CNN reported.