The US and Japan planned to start a joint military drill Monday in seas near the southern island of Okinawa, the Japanese government said, dpa reported.
The exercise scheduled through November 16 is aimed at beefing up defences of Japan's south-western islands, the Defence Ministry said.
Japan's military on Sunday dispatched destroyers and an Aegis cruiser to the US White Beach Base in Okinawa, public broadcaster NHK reported.
About 47,000 personnel, including 10,000 from the US military, are participating and a US aircraft carrier is also taking part, it said.
The drill was originally meant to practice landing on an island in Okinawa prefecture, but it was changed to training at sea because of concerns that landing practice might escalate tensions between Japan and China, the report said.
Anti-Japan protests erupted in many Chinese cities in mid-September after the Japanese government's purchase of three disputed islets from a private owner.
The Japanese-administered Senkaku Islands in the East China Sea are claimed by China and Taiwan, where they are known as Diaoyu and Tiaoyutai, respectively