The U.S.-Japan alliance is more important than ever as both nations work to curb North Korea's missile and nuclear threats, an annual foreign policy report released Tuesday says.

North Korea's buildup of nuclear arms and missile capabilities is "a grave and imminent threat" to the peace and stability of Japan and the international community and closer coordination with the United States and South Korea is needed to deal with the threat, according to the Diplomatic Bluebook 2018.

Reflecting a recent thaw, the report said momentum has been building for improved Japan-China ties, noting the need for the world's third- and second-largest economies to work in conjunction to see off global challenges.