The exercises, called "Yama Sakura," involve about 1,500 U.S. troops and 4,500 Japanese military personnel. Yama Sakura, being held on the southwestern island of Kyushu, is the biggest annual joint maneuver held with Japan’s army.

Lt. Gen. Shunzo Kizaki, the commander of the Japanese troops in the exercises, said they involve simulations of ballistic missile attacks, special forces warfare and an invasion of Japan’s southernmost main island. He said further details are classified.

Yama Sakura, a command post exercise, is mostly done around computer simulations conducted at established bases, rather than real-world deployment of troops.

Kizaki said the exercises — which run through Feb. 3 — are not directed at any particular threat, but contribute to Japan’s overall ability to deter an attack and defend its territory.

Japan has grown increasingly concerned about its defenses in its southwest, and particularly around the Okinawan islands, because of a number of incursions into its sea lanes by Chinese warships, including the movement of a Chinese flotilla through the Miyako Strait last April.

In response, the government has announced that it will bolster its monitoring capabilities in the region, and is reportedly considering boosting its submarine fleet.

Japan is also concerned over possible aggression from neighboring North Korea.

Lt. Gen. Benjamin Mixon, commander of the U.S. Army Pacific, said the exercises are designed to enhance the troops’ ability to fight together, and demonstrate the U.S. resolve to support the security interests of Washington’s allies.

"Many countries throughout the region face increasing security challenges and transnational threats," he said.

Under a mutual security treaty, about 50,000 U.S. troops are deployed throughout Japan, including major Air Force, Marine and Navy units. The U.S. Army component is smaller, but trains intensely with its Japanese counterpart.