"The cause of the accident is yet to be known. Two Chinese naval rescue vessels are heading to the scene," it said in a brief dispatch.
The dispute over the Japanese-held islets, where Chinese and Japanese ships regularly engage in a cat-and-mouse game, has raised fears of a clash between Asia's biggest powers that could even drag in the United States.
China's proclamation last November of an air defence zone covering large parts of the East China Sea has raised concerns that a minor incident could quickly escalate.
China also claims almost the entire South China Sea, rejecting rival claims to parts from Vietnam, the Philippines, Taiwan, Malaysia and Brunei in one of Asia's most intractable disputes and another possible flashpoint.