Mattis said that the US effort to counter the nuclear missile ambitions of North Korean leader Kim Jong Un is being "diplomatically led".
Mattis said the tragedy of war is well-known and "it doesn't need another characterization beyond the fact that it would be catastrophic".
"My responsibility is to have military options if needed", Mattis told a group of reporters in Mountain View, California, where he was visiting a new Department of Defense arm created to speed the integration of new technologies into the US military.
North Korea announced that military leaders are planning to launch four intermediate-range Hwasong-12 missiles only 25 miles off the coast of Guam, a USA territory in the Pacific Ocean.
USA missile shield not certain to protect Guam from North Korea
But it wasn't known until now that the contacts have continued, or that they have broached matters other than USA detainees. In 2013, the Army sent a missile defense system to Guam called Terminal High Altitude Area Defense or THAAD.
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. The country warned that the U.S.
"While our State Department is making every effort to resolve this global threat through diplomatic means, it must be noted that the combined allied militaries now possess the most precise, rehearsed and robust defensive and offensive capabilities on Earth", Mattis said.
Secretary of State Rex Tillerson told reporters on Wednesday that Americans had "nothing to fear" from Trump's remarks, and that they had instead been an attempt to use language that Kim Jong Un "would understand".
"I don't tell the enemy in advance what we are going to do", he said, adding that in the event of an attack, "We are ready".
The United Nations Security Council also unanimously voted to characterize North Korea's statements as a "threat to the world's community". "How often do you see France, China, Russia and the USA ... voting unanimously on any issue?"