MISAWA AIR BASE, Japan -- The U.S. Air Force has deployed two of its most advanced long-distance surveillance drones to a base in northern Japan over the past week, enhancing its ability to monitor nuclear activities in North Korea and Chinese naval operations.

The deployment of the two unarmed Global Hawk drones to Japan, a key U.S. ally, is intended to demonstrate Washington's commitment to security in Asia as part of its rebalancing of forces to the Pacific. But it will likely rankle with China and North Korea, which have been working to improve their own unmanned aircraft fleets.

Lt. Gen. Sam Angelella, commander of U.S. Forces Japan, said Friday the drones will remain here until October, when the typhoon season on the drones' home base on the Pacific island of Guam is over. Similar rotations from Guam to Misawa are expected in the future, though Angelella said no firm plans have been made.

He refused to comment on the specific missions the drones will carry out but noted that the Global Hawk's "capabilities are well-known."

The drone is considered particularly valuable because it can conduct long-range missions without the limitations of pilot fatigue, is able to fly at a maximum 60,000 feet and can "loiter" around any particular site of interest for 24 hours or more.

From Japan, it can easily monitor areas on the Asian mainland -- including North Korea's nuclear sites -- or targets at sea -- such as areas where China and other countries have had confrontations over territory.

The military keeps much of the Global Hawk's work secret, but Angelella spoke of its use in humanitarian missions including Japan's 2011 tsunami and the devastating typhoon that hit the Philippines last year. More recently, he said, the drone was used in surveillance work following the mass abduction of more than 300 girls in Nigeria by Islamic extremists.

The deployment of the drones will also help Japan familiarize itself with the aircraft. Tokyo plans to buy three Global Hawks.

Angelella said the aircraft has proved itself to be one of the most reliable in the Air Force. While still under development, the Global Hawk began supporting overseas contingency operations two months after the Sept. 11, 2001, attacks. As of September last year, it had surpassed 100,000 flight hours, three-quarters of which were performed in combat.

Safety is a key factor in Japan because many U.S. bases here are located in heavily populated areas.

The U.S. maintains 50,000 troops in Japan.