It is his first overseas trip since he took office less than two weeks ago.

The US' decision to select South Korea as the first stop in its defense secretary's first overseas trip clearly shows the new US government's full commitment to the defense of South Korea and its allies, Han said.

"The defense ministers' talks here in itself indicates trusted bonds between the allies and will definitely serve as a stern warning toward the provocative North," he said.

Mentioning ways to counter saber-rattling by North Korea, Mattis said, "Due to North Korea's threatening rhetoric and destabilizing behavior, we are taking defensive steps like deploying the highly effective THAAD 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 system to South Korea this year to better defend against North Korea's nuclear and missile threats.

But China has opposed the deployment on concerns that the move could hurt its strategic security interests. The US has repeatedly stressed that the system is defensive and aimed only at deterring North Korean threats.

In other efforts, Mattis stressed the need to expand "trilateral venues of cooperation with Japan, where the mutual defense of the three countries is best served through teamwork."

In November, Seoul and Japan formally signed the information sharing accord, known as the General Security of Military Information Agreement, the first military pact between them since South Korea's liberation from Japan's 1910-45 colonial rule. Washington welcomed the deal as it expects to bolster security cooperation among the three countries as a counterbalance to China's rise.

"The United States stands by its commitments -- and we stand with our allies. Our alliance is a testament to mutual commitment and respect, and it is a linchpin of peace and stability in the Asia-Pacific region," Mattis said.

He is scheduled to depart for Japan Friday afternoon after paying his respects to the National Cemetery in southern Seoul.

Tensions are running high on the Korean Peninsula after North Korean leader Kim Jong-un said in his New Year's Day address the country is close to test-firing an intercontinental ballistic missile.

Last year alone, Pyongyang conducted two nuclear tests and test-fired 24 ballistic missiles to achieve its stated goal of developing an nuclear-tipped long-range missile that could strike parts of the US mainland. (Yonhap)