Hours after President Donald Trump intensified his threat to rain "fire and fury" upon North Korea, Mattis said war would be "catastrophic."
It is the second time this week that a key member of the US administration has tried to calm the waters after bellicose comments from the President.
Secretary of State Rex Tillerson has also sought to promote dialogue and diplomacy over military action.
But the tension continued Friday when North Korea said Trump was "driving the situation on the Korean peninsula to the brink of a nuclear war" with his continued rhetoric.
Trump showed no signs of stepping back, suggesting on Twitter Friday that he had a range of military solutions to use against North Korea, should he need them.
"Military solutions are now fully in place, locked and loaded, should North Korea act unwisely. Hopefully Kim Jong Un will find another path!" he wrote.
Trump earlier offered no contrition for his remark on Wednesday that aggression by North Korea would met with "fire and fury like the world has never seen." Striking a defiant tone on Thursday evening, he told reporters: "Maybe it wasn't tough enough."
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."
Responding to a North Korea's threat to fire four missiles
in the waters off the US Pacific territory of Guam, home to US military bases, Trump vowed a tough response. "Let's see what he does with Guam," Trump said in apparent reference to North Korean leader Kim Jong Un. "He does something in Guam, it will be an event the likes of which nobody has seen before."
Speaking in California later Thursday, Mattis praised the diplomatic leadership of Tillerson and US Ambassador to the UN Nikki Haley, as well as a unanimous vote by the UN Security Council to impose fresh sanctions on North Korea.
"My portfolio, my mission, my responsibility is to have military options should they be needed. However, right now, Secretary Tillerson, Ambassador Haley, you can see the American effort is diplomatically led, it has diplomatic traction, it is gaining diplomatic results," Mattis said.
"The tragedy of war is well enough known, it doesn't need another characterization beyond the fact that it would be catastrophic."
While Mattis insists the US approach is diplomatic, little has been made public about who exactly is doing what, beyond Tillerson and Haley's high-level meetings.
In fact, the Trump administration is still yet to appoint several diplomatic roles key to the North Korea situation, which are being run by temporary officials
in an acting capacity. Among them is the post for US Ambassador to South Korea and Assistant Secretary for East Asian and Pacific Affairs.
There is also division between diplomats from other countries on Trump's comments, with some criticizing them as incendiary and others supporting them.
When asked whether Trump's "fire and fury" comments on Tuesday were inflaming the situation, the UK Ambassador to the UN Matthew Rycroft backed Trump.
"The UK stands shoulder to shoulder with the US in tackling this threat and many others. We are united in calling on the North Korean regime to halt its nuclear program. The whole of the Security Council has come together behind that, and I think that we should see the President's public comments as one of the strands, one of the many strands of US activity," he said.
But UN Secretary General António Guterres hinted that Trump's remarks risked derailing the diplomatic process.
"On the situation on the Korean peninsula obviously the Secretary General remains extremely concerned by the ongoing situation; he's troubled by the increase in confrontational rhetoric that we've seen," a spokesman for Guterres said.
"The Security Council passed a resolution unanimously over the weekend. We obviously reiterate or welcome that commitment to peaceful diplomatic and political solution to the situation. The Secretary General welcomes all initiatives that will help deescalate tensions and welcomes a return to diplomacy."
The North Korea threat has been a foreign policy priority for the Trump administration since it took office in January, but it has taken center stage since Pyongyang test fired two intercontinental ballistic missiles last month.
The issue came to the fore again this week when US intelligence analysts assessed that North Korea had produced a miniaturized nuclear warhead, according to multiple sources, which would bring the country a significant step closer to being able to fire a nuclear-tipped warhead at the US or its allies.
Pressure on China
Trump's remarks have prompted a reaction in China, where the state-controlled Global Times newspaper said in an editorial Friday that if North Korea launched missiles at the US, it should "stay neutral," sending a warning to Pyongyang that Beijing would not support the regime if it made a first move.
But it also said that if the US and South Korea took a first strike against North Korea, China should intervene.
"China should make clear that if North Korea launches missiles that threaten US territories and triggers US retaliations, China will stay neutral. We should also make clear that, if the US and South Korea carry out military attacks to overthrow the North Korean regime and change the political landscape on the Korean Peninsula, China will be determined to take actions to prevent them from doing so."
The Global Times is a state-sanctioned tabloid known for its nationalistic and often anti-West editorials, and its views do not always represent official Chinese government policies.
It also said that China and Russia should enhance coordination to let other parties know they would not stand aside should their security be threatened by the escalating tensions.