An unidentified submarine briefly entered Japan's territorial waters off the Okinawa Islands early Wednesday, and Maritime Self-Defense Force patrol aircraft were sent to track it.
Although some media reported that the Defense Agency believed it was a Chinese nuclear-powered submarine, government officials rejected such speculation and said it had yet to confirm the submerged vessel's nationality.
But a government source, speaking on condition of anonymity, did say later Wednesday it was highly likely the submarine is nuclear-powered, given its noise patterns.