Speaking at an event in California on Thursday, 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.
US Department of Defense (MOUNTAIN VIEW, Calif.) - Secretary of Defense James Mattis made his first on-camera remarks Thursday about North Korea's threats to attack Guam with ICBMs, saying that a potential nuclear incident "would be catastrophic".
"The tragedy of war is well-enough known it doesn't need another characterization beyond the fact that it would be catastrophic". And I want to stay right there right now.
The President's comments on Tuesday have been the backdrop to a week of growing tensions on the Korean peninsula.
"No Easy Military Options in North Korea" is part of Aviation Week & Space Technology's subscription package.
North Korea successfully tested an intercontinental ballistic missile last month for the first time, and US intelligence agencies assess that Pyongyang is capable of producing a compact nuclear warhead and putting it atop a long-range missile.
He told reporters that the diplomacy effort spearheaded by Secretary of State Rex Tillerson and U.N. Ambassador Nikki Haley "has [gained] diplomatic traction" and "is gaining diplomatic results", according to the BBC.
"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", he said in a statement.
"If you threaten a nation, then what should you expect - a stiffly worded letter that would be sent by courier?" he said, in response to a question on whether the situation could escalate into a military conflict.