Speaking at an event in California, Mattis said his mission and responsibility was to have military options ready "should they be needed" but stressed the US effort is currently focused on diplomacy.
"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.
"The tragedy of war is well-enough known it doesn't need another characterization beyond the fact that it would be catastrophic," he added.
Mattis's comments came shortly after President Donald Trump warned North Korea it should be "very, very nervous" if it thinks of attacking America, and doubled down on a vow to rain "fire and fury" on Pyongyang if it continued to threaten the US.
Mattis did not elaborate on the diplomatic results he referenced, but the UN Security Council at the weekend passed a new set of sanctions against Pyongyang over its weapons program, including bans on the export of coal, iron and iron ore, lead and lead ore as well as fish and seafood.
In a diplomatic win for the United States, the measures were approved unanimously -- including by Russia and China, the North's sole major ally.
The Pentagon chief on Wednesday issued a carefully worded statement telling North Korea it would be "grossly" outmatched in any conflict with the US, and warning Pyongyang to stop considering any action that risked "the destruction of its people."
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