The comment came after Pyongyang elevated the crisis over its growing potential nuclear threat with the test launch of an intermediate range ballistic missile
"We're never out of diplomatic solutions," Mattis said as he went into a meeting with South Korea Defense Minister Song Young-Moo.
"We continue to work together and the minister and I share responsibility to provide for the protection of our nation our populations and our interests, which is what we are here to discuss," he added.
"We are never complacent."
The comment came after Pyongyang elevated the crisis over its growing potential nuclear threat with the test launch of an intermediate range ballistic missile over Japanese territory on Tuesday.
Before Mattis spoke, Trump had tweeted: "The U.S. has been talking to North Korea, and paying them extortion money, for 25 years. Talking is not the answer!"
North Korea leader Kim Jong-Un has promised more missile flights over Japan, insisting his nuclear-armed nation's launch was a mere "curtain-raiser," in the face of UN condemnation and US warnings of severe repercussions.
The Hwasong-12 intermediate-range missile unleashed by Pyongyang represented a major escalation of tensions over its weapons programs.
The missile has the potential of reaching the US military base at Guam in the Pacific. In July, North Korea test-launched an intercontinental ballistic missile that theoretically could carry a nuclear bomb to the continental United States.
After the latest launch, Trump said that "all options" were on the table, reviving his implied threat of pre-emptive US military action just days after congratulating himself that Kim appeared to be "starting to respect us."
On Tuesday, the UN Security Council -- which has already imposed seven sets of sanctions on Pyongyang -- unanimously condemned the North's "outrageous" actions, calling them "not just a threat to the region, but to all UN member states."
North Korea must "abandon all nuclear weapons and existing nuclear programs in a complete, verifiable and irreversible manner, and immediately cease all related activities," the Security Council said.