Speaking at an event in California yesterday, Mattis said his mission and responsibility was to have military options ready "should they be needed" but stressed the U.S. effort is now focused on diplomacy.
U.S. Defense Secretary Jim Mattis: North Korea "would lose any arms race or conflict it initiates".
The North Korean government said Thursday its attack plans would be finalized within the next week or two.
"The American effort is diplomatically led, it has diplomatic traction, it is gaining diplomatic results and I want to stay right there right now", Mattis said at an event in California.
Mattis says the tragedy of war is well known and "it doesn't need another characterization beyond the fact that it would be catastrophic".
President Donald Trump "was informed of the growing threat last December and on taking office his first orders to me emphasized the readiness of our ballistic missile defense and nuclear deterrent forces", Mattis said.
"It's [North Korea's] aligning the United Nations in very serious sanctions, and I would just tell you that it did not happen by accident", Mattis said.
Speaking on Thursday at his golf club in Bedminster, New Jersey, Mr Trump suggested his own statements on North Korea - where he threatened them with "fire and fury the likes of which the world has never seen" - had not been tough enough.
However, he added that the U.S. would always consider negotiations.
Mattis said that "the United States and our allies have the demonstrated capabilities and unquestionable commitment to defend ourselves from an attack".