Story highlights Defense Secretary James Mattis makes first overseas trip as Pentagon chief

He says he'll listen to South Korean leaders before deciding on US policy toward North

(CNN) New US Defense Secretary James Mattis on Thursday defended the planned deployment of a missile defense system in South Korea, saying only North Korea had anything to fear from it.

After arriving at the Osan Air Base outside Seoul, Mattis said Pyongyang's "provocative behavior" was the only reason the US-developed system was on the agenda.

South Korea's decision to deploy the Terminal High Altitude Area Defense anti-missile system, known as THAAD, has drawn sharp criticism from China, which sees it as part of a broader US strategy to extend its military alliance network from Japan all the way down to the South China Sea.

"There is no other nation that needs to be concerned about THAAD other than North Korea if they're engaged in something that's offensive," Mattis said.

On Thursday afternoon, Mattis met with acting South Korean President Hwang Kyo-ahn in Seoul and reiterated the strength of the US-South Korean alliance.