China is attempting to open a new front in its territorial dispute with Japan by questioning Tokyo's sovereignty over the island of Okinawa, home to 25,000 US troops.

The two countries are already pushing rival claims to the Senkakus, a chain of uninhabited islands in the East China Sea that are controlled by Tokyo. The dispute over the islands, known as the Diaoyu in China, has hit bilateral trade and sent diplomatic relations to their lowest point for decades.

Beijing began its attempt to broaden the territorial dispute earlier this month when the communist party newspaper, the People's Daily, ran an article in which two Chinese academics challenged Japan's sovereignty over the Ryukyu chain of islands, which includes Okinawa.

Luo Yuan, a two-star general in the People's Liberation Army, raised the territorial stakes again this week, saying the Ryukyus had started paying tribute to China in 1372, half a millennium before they were seized by Japan.

"Let's for now not discuss whether [the Ryukyus] belong to China, they were certainly China's tributary state," Luo said in an interview with China News Service. "I am not saying all former tributary states belong to China, but we can say with certainty that the Ryukyus do not belong to Japan," he added, in comments translated by the South China Morning Post.

The potential for more diplomatic clashes over territory comes amid fresh criticism of Japan's attitude towards its wartime conduct in China and the Korean peninsula.

Beijing reacted angrily after the outspoken nationalist mayor of Osaka, Toru Hashimoto, said this week that Japan's forced recruitment of Asian women to work in military brothels before and during the second world war had been necessary to maintain discipline among soldiers.

"We are appalled and indignant about the Japanese politician's comments boldly challenging humanity and historical justice," Hong Lei, a Chinese foreign ministry spokesman, told reporters.

"The way they treat the past will determine the way Japan walks toward the future. On what choice Japan will make, its Asian neighbours and the international community will wait and see."

On Wednesday Hashimoto attempted to clarify his remarks, saying he had not sought to justify the use of so-called comfort women, but was questioning why Japan had been singled out for criticism given that other countries had, he said, operated similar schemes.

Okinawa, an island of more than 1 million people, hosts more than half the 47,000 US troops stationed in Japan.

Washington and Tokyo have agreed to reduce Washington's military footprint on Okinawa, but the island is seen as key to the US's ability to respond quickly to maritime provocations by the increasingly robust Chinese navy, as well as a crisis on the Korean peninsula.

In their People's Daily article, Li Guoqiang and Zhang Haipeng, prominent academics at the government-run Chinese Academy of Social Sciences, said Japan's annexation of the Ryukyu kingdom in 1879 amounted to an invasion, and that the sovereignty issue remained open to question.

They pointed out that the kingdom had previously been a Chinese vassal state, adding that the ruling Qing dynasty had been too weak to resist Japan's advance.

"Hanging in the balance of history, the unresolved problem of the Ryukyus has finally arrived at the time for reconsideration," they wrote.

In a later article for the paper's sister publication, the Global Times, Li said: "Not only is Japan obliterating the truth about the Ryukyu issue, but it is doubling its aggressiveness and making provocations over the Diaoyu issue. Therefore it is necessary to revisit the Ryukyu issue."

China's foreign ministry dismissed Japanese protests over the article.

Hua Chunying, a ministry spokeswoman, told reporters that China "does not accept Japan's representations or protests". She said the article reflected renewed interest in the islands in the light of Japan's provocative actions over the Senkakus.

Japan's government effectively nationalised three of the disputed islets after buying them from their private owners last year, sparking violent protests across China and forcing the temporary closure of Japanese businesses in the country.

Okinawa, scene of one of the bloodiest battles of the Pacific war, was controlled by the US until it was returned to Japan on 15 May 1972.

The continued presence of US troops and military hardware is a constant source of tension with the civilian population, who complain about crimes by soldiers, noise pollution and the potential for accidents involving aircraft.

Analysts said China was mistaken if it believed that provoking Japan over Okinawa would add momentum to its claims to the Senkaku islands. "If China's goal is to hold talks with Japan over the Senkakus, articles like these are counterproductive," M Taylor Fravel, a Chinese foreign policy expert at the Massachusetts Institute of Technology, told Associated Press.

"As a result, Japan has an even stronger incentive now to stand firm with China and not hold talks."