U.S. Secretary of State John Kerry joined Defense Secretary Chuck Hagel on a visit to Japan. Kerry said in a statement the delegation was there to express its "serious commitment" to shifting its strategic focus to the region.
The U.S. military started shifting more of its assets to the region after combat obligations in Iraq ended in 2011.
Kerry said Thursday both sides agreed to revisions in their bilateral defense relationship to "account for growing threats, including from North Korea and the threat to maritime security."
Japanese Foreign Minister Fumio Kishida said Thursday the developments in the region were threatening the status quo. Apart from North Korea, the Japanese government and Beijing are at odds over a small chain of islands in the East China Sea.
The Defense Department said Thursday it would deploy a second radar installation in Kyoto to defend against any potential missile threats from North Korea. Pyongyang conducted a nuclear test this year and touts missiles it says could reach the western U.S. coast.
"The United States-Japan relationship has underwritten the peace, stability, and prosperity of the Asia Pacific region for more than a half century," Hagel said in a statement. "Today, we have helped ensure this alliance continues to do so in the 21st century."