According to the source, the Japan Maritime Self-Defense Force detected the presence of a Chinese Han-class nuclear-powered submarine and a Song-class diesel attack submarine in the region as the U.S. George Washington nuclear aircraft carrier was heading to the South Korean port of Pusan on a friendly visit.
The USS George Washington is stationed at a U.S. naval base in Yokosuka, Japan.
The source said the Chinese submarines did not violate Japanese territorial waters, but described their activities as attempts "to gather data on noise signatures emitted by all automated systems of the U.S. aircraft carrier" and "some sort of intimidation."
In response to the Chinese moves, Tokyo and Washington increased the number of patrol flights by Japan's P-3C Orion ASW aircraft and deployed additional U.S. reconnaissance satellite assets to monitor the area.
The Chinese Navy's submarine fleet includes at least two nuclear-powered attack submarines, and its 13 Song-class submarines are extremely quiet and difficult to detect when running on electric motors.
In October 2006, an undetected Chinese Song-class submarine popped up in the middle of a U.S. task force during an exercise in the Pacific at the distance of only 5 miles from the USS Kitty Hawk aircraft carrier.
The Chinese vessel slipped past at least a dozen other American warships which were supposed to protect the carrier from hostile aircraft or submarines.