Japan is revising its middle and high school teaching guidelines to bolster its claim to a series of disputed islands also claimed by South Korea and China.
Education Minister Hakubun Shimomura said Tuesday that under the revision teachers will be required to tell their students the islands are "integral territories of Japan."
The move applies to a group of small islets in the Sea of Japan, known in Japanese as Takeshima and in South Korean as Dokdo. It also applies to the uninhabited East China Sea islands, called Senkaku in Japan and Diaoyu in China.
In response, South Korea summoned Japan's ambassador to Seoul. South Korea's foreign ministry spokesman Cho Tai-young threatened "firm measures" against Japan.
"Our government strongly condemns this (action of modifying Japanese textbook guidelines), and demands that they withdraw this immediately. If the Japanese government does not follow this, our government will take firm measures that corresponds to this right away."
China was also upset at the move. Foreign ministry spokeswoman Hua Chunying urged Japan to "respect historic realities."
"We express serious concern over this and have already lodged a solemn protest to Japan. I want to stress that Diaoyu islands and the adjacent islands have been inherent Chinese territory since ancient times. No matter what the Japanese side is deliberately plotting to promote its false claims by various means, it's absolutely impossible to change the basic fact that the Diaoyu Islands belong to China.''
Japan's ties with South Korea and China had already been suffering. Both countries were victims of Japan's imperial aggression before World War Two and have complained Japan is not doing enough to atone for its past.
Japanese Prime Minister Shinzo Abe upset Seoul and Beijing late last year by visiting a controversial war shrine that includes several convicted war criminals among the Japanese war dead that it honors.
Mr. Abe, who took power for the second time in 2012, has taken a more hawkish stance on territorial issues and vows to change his country's pacifist constitution to take on an expanded regional role.