US Defense Secretary Jim Mattis said that none other than North Korea needs to worry about the planned deployment of the US THAAD missile defense system in South Korea.

Mattis, who arrived in Seoul on his first overseas trip since taking office, made the remark to reporters traveling together with him on the plane, stressing THAAD is a defensive system, according to a transcript provided by the Pentagon.

"There's only one reason that we even have this under discussion right now, and that is North Korea's activities," Mattis said. "THAAD is for defense of our allies' people, of our troops who are committed to their defense."

"Were it not for the provocative behavior of North Korea we would have no need for THAAD out here. There's no other nation that needs to be concerned about THAAD other than North Korea," he said.

The remark was targeted at China that has strongly denounced the decision by the South and the US to deploy THAAD, claiming the system poses threats to its security, despite repeated assurances from Washington that the system is designed only to defend against the North.

Mattis is expected to reaffirm THAAD's deployment in his talks with South Korean Defense Minister Han Min-koo Friday.

Upon arrival in Seoul, Mattis held meetings with South Korea's Acting President and Prime Minister Hwang Kyo-ahn and National Security Adviser Kim Kwan-jin.

During the meetings, the Korean officials impressed upon Mattis the importance of close cooperation in the face of North Korean threats, and Mattis responded the "alliance would continue to take defensive measures in response to these threat developments, such as the stationing of THAAD to Korean Peninsula," according to Pentagon spokesman Cmdr. Gary Ross.

"The secretary emphasized the priority that President Trump places on the Asia-Pacific, and on strengthening the US-ROK alliance. He also underscored that the United States remains steadfast in its commitment to defend the ROK and that the US extended deterrence commitment remains ironclad," Ross said in a statement.

Mattis also said he inherited an already strong alliance, but will make it even stronger, according to the statement.

The trip came amid heightened tensions on the Korean Peninsula after North Korean leader Kim Jong-un said the country is close to test-firing an intercontinental ballistic missile, an apparent threat the country is perfecting capabilities to strike the US with nuclear weapons.

Asked if he believes the North is close to an ICBM test, Mattis said en route to Seoul, "North Korea has often acted in a provocative way, and it's hard to anticipate what they do. It's hard to anticipate what they do."

"One of the reasons I want to come out and talk to the leadership out here, they live in the neighborhood. They watch this as an existential to them, and I need to get to some -- some data from them. I need to get their appreciation of the situation before I start making statements on where I stand on it," he said.

It is Mattis' first overseas visit since taking office less than two weeks ago. That underscores the seriousness and urgency with which the retired Marine Corps general takes the threats from North Korea, as well as the importance of the alliances with South Korea and Japan in dealing with the provocative regime.

"Basically, what we're doing is what we talked about earlier today, it's a priority for the President Trump's administration. We pay attention to the Northwest Pacific, to our two strong allies," Mattis said of the trip to South Korea and Japan.

"Two of the most enduring alliances that we have had have been between Japan and us, and of course South Korea and the United States, and together we confront the North Korean situation," he said. "I want to listen to them, engage with their political leaders, listen to some of their briefs, get an understanding of their view of the situation."

Mattis said South Korea is not new to him, saying he was in the country as a young officer in 1972.

"But I've been not back out here in a long, long time. So I'm going to get current by listening to them, finding out where their issues are, and then we're going to work together and strengthen our alliance," he said. (Yonhap)