Japan's navy told the transportation ministry that the US Air Force jets came as close as 450 meters (yards) to a Beechcraft aircraft last Thursday over northern Aomori prefecture.

"I take allegations like this very seriously," Air Force Lieutenant General Bruce Wright, the commander of US forces in Japan, said in a statement.

"My overall objective in launching a full investigation into this allegation is to ensure our pilots are conducting safe operations at all times," he said.

"I also want to validate that our highly trained and professional airmen are adhering to all established safety standards and regulations."

Some 40,500 US troops are stationed in officially pacifist Japan under a security alliance and have frequently come under criticism from local residents over their behavior on and off the job.

Senior US commanders repeatedly go to pains to show they are sensitive to the concerns.

In July, the military imposed a curfew on Kadena Air Base, the largest US military hub in Asia, after a serviceman at the base on the southern Okinawa islands allegedly groped a 10-year-old Japanese girl.