But it has been given new attention by the row over the uninhabited islets, known as the Diaoyu islands in China, which claims them, and as the Senkaku chain in Japan, which controls them.

In recent anti-Japan protests in China, some demonstrators carried signs reading: "Retake Ryukyu" and "Take back Okinawa".

China's government does not make such claims, but state media have carried articles and commentaries questioning Japan's authority.

In an article carried by state media in July, People's Liberation Army Major General Luo Yuan wrote: "The Ryukyu Kingdom had always been an independent kingdom directly under the Chinese imperial government before it was seized by Japan in 1879."

The kingdom, which lasted from 1429 until 1879, had a complex history wedged between powerful neighbours.

In return for tribute to Chinese emperors, trade and cultural ties flourished. But from the early 17th century, it came under pressure from Japan, suffering a punitive invasion and demands for loyalty and tribute.

Nominal independence, however, was maintained, and the "dual subordination" continued until the late 19th century when a modernising Japan could no longer tolerate Ryukyu's vague status.

Western and Japanese scholars say Okinawa's links to China are no basis for sovereignty claims today. Many states were part of a China-centred structure of international relations in Asia.

"It was a system of cultural subordination and also a way of the Chinese empire attempting to control trade," said Gregory Smits, an expert on Ryukyu history at Pennsylvania State University.

Experts see little chance of Beijing pushing a demand for Okinawa.