BEIJING - China criticised Japan on Thursday for lodging a diplomatic protest against a Chinese state media commentary calling into question Japanese sovereignty over the southern Ryukyu Islands, which include Okinawa.

The latest angry exchange could further strain tense relations between Asia's two-largest economies, which are involved in a stand-off over a group of uninhabited islands in the East China Sea, called the Senkaku in Japan and Diaoyu in China.

On Wednesday, the People's Daily, the Chinese Communist Party's mouthpiece, published a commentary by two Chinese government-backed scholars who said ownership of the Ryukyu islands should be re-examined, prompting Japan to lodge the diplomatic protest.

"China cannot accept Japan's so-called negotiations or protests," Chinese Foreign Ministry spokeswoman Hua Chunying said at a regular briefing.

"The relevant scholars' academic articles reflect attention and research paid by China's populace and academia to the Diaoyu Islands and related historical problems," Hua said.

Japan's Chief Cabinet Secretary Yoshihide Suga told a regular briefing in Tokyo on Wednesday that the islands were Japanese territory.

"Japan lodged a stern protest that we can by no means accept the article in question if it reflects the Chinese government's stance," Suga said.

China had responded to Japan by saying that the piece was written by scholars as individuals, Suga said.

Okinawa, host to the bulk of up to 50,000 US military personnel in Japan, is the largest island in the Ryukyu chain, which extends south towards Taiwan.

The scholars from a top government think-tank, the Chinese Academy of Social Sciences, called the Ryukyu Islands a "vassal state" of China's Ming and Qing dynasties before they were annexed by Japan, suggesting that China had a historical claim to the island chain.

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

Chinese-Japanese relations plummeted to their lowest point since normalisation of ties more than 40 years ago when Japan bought disputed the East China Sea islets from a private Japanese owner in September.

That row has in recent months escalated to the point where both sides have scrambled fighter jets while patrol ships shadow each other in nearby seas, raising worry that an unintended collision or other incident could lead to a broader clash.