U.S. Defense Secretary Mark Esper said on Monday that the United States is committed to ensuring a "free and open" Indo-Pacific region along with its partners, including South Korea, amid challenges from China.

The defense chief made the remarks in an opinion piece for the Singapore-based daily The Straits Times.

In the article titled "US stands with its partners for a free and open Indo-Pacific," Esper said the U.S. continues to build closer relationships with its Indo-Pacific allies and partners through security cooperation, information sharing and exercises.

He said this includes efforts alongside South Korea to achieve "the final, fully verified denuclearization" of North Korea and an enduring peace on the Korean Peninsula.

The U.S. has not used this expression for a while and the remarks come amid heightened tensions on the Korean Peninsula.