"America's commitments to defending our allies and to upholding our extended deterrence guarantees remain ironclad: any attack on the United States, or our allies, will be defeated, and any use of nuclear weapons would be met with a response that would be effective and overwhelming," Mattis told the media here.
Mattis made the remarks during a press briefing before he began talks with South Korean Defence Minister Han Min-koo, to discuss ways to maintain stability on the Korean Peninsula and in the region, Yonhap News Agency reported.
The retired Marine Corps general arrived in Seoul on Thursday for a two-day visit for talks with top South Korean officials.
On Thursday, he met with South Korea's acting President and Prime Minister Hwang Kyo-ahn to underscore the "priority" that the US President Donald Trump's administration places on the Seoul-Washington alliance.
It is his first overseas trip since he took office less than two weeks ago.
Mentioning ways to counter saber-rattling by North Korea, Mattis, on Thursday said: "Due to North Korea's threatening rhetoric and destabilising behaviour, we are taking defensive steps like deploying the highly effective Terminal High Altitude Area Defence anti-missile unit to South Korea to protect its people and our troops that stand with our ally."
In July 2016, Seoul and Washington agreed to deploy the THAAD anti-ballistic missile system to South Korea this year to better defend against North Korea's nuclear and missile threats.
In other efforts, Mattis stressed the need to expand "trilateral venues of cooperation with Japan, where the mutual defence of the three countries is best served through teamwork."
Mattis is scheduled to depart for Japan on Friday afternoon after paying his respects at the National Cemetery in southern Seoul.