Japan has sent a "strong protest" to China after a Communist party-run newspaper published a story questioning Tokyo's ownership of the southern prefecture of Okinawa.
The People's Daily on Wednesday quoted experts at a state-run research organization who say it is time to reconsider Japan's sovereignty over the Ryukyu island chain, which includes Okinawa.
Japanese Chief Cabinet Secretary Yoshihide Suga Thursday said it would be "absolutely unacceptable" if the article reflected the opinion of the Chinese government. He says the Chinese side responded that the view in the commentary was solely held by the researchers.
Sino-Japanese relations are already tense because of a recent flare-up in a dispute about a tiny but strategic island chain in the East China Sea.
China has regularly sent patrol vessels near the Japanese-controlled islands, raising fears of a military conflict. The uninhabited islands, known as Senkaku in Japan and Diaoyu in China, are surrounded by rich fishing grounds and possibly by energy deposits.
The People's Daily article repeats China's arguments for ownership of the island chain, which was annexed by Japan in the late 19th century, but claimed by China in 1971.
Okinawa, which lies farther north, hosts a large U.S. military presence. It is home to 1.3 million people.