While speaking to reporters at the Defense Innovation Unit Experimental (DIUx) event in Mountain View, California, he was asked by a reporter, "Can you talk about the human toll we might see in the event of a nuclear confrontation?"
"My portfolio, my mission, my responsibility is to have military options should they be needed".
Hours after President Donald Trump intensified his threat to rain "fire and fury" upon North Korea, Mattis said war would be "catastrophic".
Mr Mattis said any action by North Korea would be grossly overmatched by the U.S., and that Pyongyang would lose any arms race or conflict it started.
"The tragedy of war is well enough known". He said Kim Jong Un's regime should listen to and "act on that clarity". "I'm quite proud of what we've accomplished".
But UN Secretary General António Guterres hinted that Trump's remarks risked derailing the diplomatic process.
French government spokesman Christophe Castaner said Paris is "watching what's going on with concern" and "is ready to put all its means at disposal so we can find a peaceful solution".
"The Security Council passed a resolution unanimously over the weekend".
"We want to use diplomacy". "That shows where the Trump administration goes in terms of the prioritizing of the threat but also how to deal with it in a diplomatically effective manner".
There are about 29,000 American troops permanently stationed in South Korea.
And he points to another sign that the U.S.is not moving toward war with North Korea: There have been no efforts to evacuate the at least 150,000 USA citizens living in South Korea.
Diplomats from each country, including USA envoy to North Korea Joseph Yun and North Korean diplomat Pak Song Il are maintaining a relationship with each other in NY, fostered as well by outside groups that favor direct engagement.
It also said that China and Russian Federation should enhance coordination to let other parties know they would not stand aside should their security be threatened by the escalating tensions.