In a glass case at Beijing's Imperial College, an 18th century book with a yellowed title page in bold, black characters is evidence -- some Chinese say -- that a swathe of modern-day Japan belongs to China.

The two Asian powers are already at loggerheads over a set of tiny uninhabited islets in the East China Sea, even stoking fears of armed conflict.

But the most aggressive Chinese nationalists -- tacitly encouraged by authorities -- say far more is open to claim, including the island of Okinawa, home to 1.3 million people and major US military bases.

The biggest of the Ryukyu Islands, which stretch for about 1,000 kilometres (620 miles) from Japan's mainland almost to Taiwan, Okinawa was the centre of the Ryukyuan kingdom, which pledged fealty to both Chinese emperors and Japanese feudal lords.

For hundreds of years it paid tribute to China's Ming and Qing dynasties, until it was absorbed by Japan in 1879.

The people of the Ryukyus are considered more closely related to Japan in ethnic and linguistic terms, than to China.

Some Chinese, however, see historical and cultural ties as a basis for sovereignty and dismiss Japan's possession of the islands as a legacy of its aggressive expansionism that ended in World War II defeat.

"This kind of thing proves Ryukyu is China's," said electrical engineer Zhu Shaobo, looking at a Qing dynasty volume from the 1760s about Ryukyuan students on display at the Imperial College, now a tourist site.

"Ryukyuan students studied hard and the cultural level of some was not inferior to Chinese students," explains an exhibit panel at the institution, which trained Imperial officials and some foreign students.

The belief that China has a legitimate claim to the Ryukyu Islands has existed among flag-wavers in China -- and Taiwan -- for years.

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.

Gavan McCormack, emeritus professor at Australian National University, called any claim "quite unrealistic", adding it was probably "an extreme position to try and attract Japan back to the negotiating table".

Jia Qingguo, an international relations expert at Peking University, added: "I don't think the Chinese government wants to further complicate the already complicated issue."

Still, questions being raised over Japan's sovereignty worry Akihiro Kinjo, a 25-year-old Okinawa native and restaurant manager in Beijing.

Okinawa was the site of a deadly 1945 battle between Japan and the United States.

"Our grandmothers experienced war and based on their stories they had a horrible time," he said.

Under a security treaty with Japan the US maintains major military facilities on the island, and Washington is also making a strategic "pivot" to Asia, raising fears in Beijing of containment.

In August, the World Journal, a military affairs tabloid, carried a cover showing a projectile soaring toward Okinawa under the headline: "People's Liberation Army guided missile targets Okinawa bases."