China says its claim extends back hundreds of years. Japan says it saw no trace of Chinese control of the islands in an 1885 survey, so formally recognized them as Japanese sovereign territory in 1895. Japan then sold the islands in 1932 to descendants of the original settlers. The Japanese surrender at the end of World War II in 1945 only served to cloud the issue further.

The islands were administered by the U.S. occupation force after the war. But in 1972, Washington returned them to Japan as part of its withdrawal from Okinawa.

Taiwan, which Beijing regards as a breakaway province, also lays claim to the islands. But the self-governing island has seldom rigorously advanced its claims because of an unwillingness to risk its good relationship with Japan, said Alan Dupont, a strategic analyst at the University of New South Wales in Sydney.

He said last year that Taiwan's decision to become more assertive was a response to actions taken by China and Japan in the second half of 2012, as well as concerns over access to fishing and marine resources.

Last week, Japanese Coast Guard ships fired water cannons and shouted warnings at a boat carrying activists from Taiwan who were attempting to land on the islands.