Story highlights US Defense Secretary heads to Asia on his first official trip

US and Korean Ministers agreed over phone call to strengthen existing defense alliance

(CNN) Secretary of Defense Jim Mattis has reaffirmed the US' commitment to defend South Korea "against the evolving North Korean threat".

Mattis spoke with the South Korean Defense Minister Han Minkoo Tuesday ahead of his inaugural trip as secretary of defense to visit Japan and South Korea, the US' closest Asian allies.

Han and Mattis expressed their commitment to proceed with the THAAD missile defense system deployment and the need for closer co-operation on policies on North Korea, according to a statement released by the South Korean Ministry of Defense.

The THAAD system is designed to take down incoming projectiles and has been cited as a way to potentially stop a nuclear attack from North Korea.

The visit comes after Donald Trump, when campaigning for the US presidency, caused concern in Asia by suggesting Seoul and Tokyo develop their own nuclear weapons. Trump also suggested the two countries either pay more for their own defense, or the US provide them with less security support.

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