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 USA effort is now focused on diplomacy.
And while Mattis warned on Wednesday that North Korea would be "grossly overmatched" by the USA 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.
North Korea said on Wednesday it was considering a missile attack on the U.S. island of Guam, which hosts several U.S. military bases.
"The tragedy of war is well enough known", Mattis said, responding to a question about the human toll of a nuclear confrontation between the DPRK and the U.S.
Australia's prime minister says his country would come to the aid of the United States if North Korea attacks Guam.
Johnson: UK working to bring diplomatic end to North Korea crisis
At the same time, a North Korean state news agency statement warned that "even a single shell dropped on the Korean Peninsula might lead to the outbreak of ... thermonuclear war".
It's the latest warning since he said earlier this week that North Korea faces "retaliation with fire and fury unlike any the world has seen before".
North Korea Gen. Kim Rak Gyom said Trump is "extremely getting on the nerves" of his soldiers by making comments that showed his "senility" again.
"The UK stands shoulder to shoulder with the U.S.in tackling this threat and many others".
In a lengthy exchange with reporters at his Bedminster golf resort, he said past administrations had not done enough to take on North Korea and that it was time a president "stuck up for the country".
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Mattis also noted that the U.S.is also ready for a military conflict if it comes to that. "The Secretary General welcomes all initiatives that will help deescalate tensions and welcomes a return to diplomacy".
Tension in the region has risen since the reclusive North, which staged two nuclear bomb tests previous year and launched two intercontinental ballistic missile tests in July in defiance of world powers.
Meanwhile North Korea's state-run KCNA news agency said its army would complete plans in mid-August to fire four intermediate-range missiles over Japan to land near Guam.
The announcement Thursday warned that the North is preparing a plan to fire four of its Hwasong-12 missiles over Japan and into waters around the tiny island, which hosts 7,000 USA military personnel on two main bases and has a population of 160,000.
Trump discusses North Korea with France's Macron
The leaders also discussed the need to confront the "increasingly unsafe situation" with Pyongyang's destabilizing behavior. Asked Friday if the USA was going to war, he said cryptically, "I think you know the answer to that".