Japanese education chiefs will instruct schools to teach children that islands at the centre of disputes with China and South Korea belong unequivocally to Tokyo, the government said yesterday, prompting an angry response from Seoul.
South Korea called in the Japanese ambassador to lodge a formal protest over the revisions.
In a separate statement, the South Korean foreign ministry threatened unspecified "reciprocal countermeasures" if the revisions were not withdrawn.
There was no immediate response from China.
The revised teachers' manuals for junior and senior high schools will be issued to education boards across the nation, a Japanese education ministry official said.
"From the educational point of view, it is natural for a state to teach its children about integral parts of its own territory," Education Minister Hakubun Shimomura told a news conference.
The move comes after Prime Minister Shinzo Abe has repeatedly stirred controversy with his unabashed nationalism, which has included visiting the Yasukuni war shrine, widely viewed by neighbouring countries as a symbol of Tokyo's wartime aggression.
Japan is embroiled in a row with China over the Tokyo-controlled Senkaku islands in the East China Sea, claimed as the Diaoyus by Beijing.
The dispute regularly sees stand-offs between paramilitary ships and has also involved military vessels and planes.
Some observers say the islands represent a key fault line for the region and could be the spark for an armed conflict.
Tokyo and Seoul, meanwhile, are at odds over the sovereignty of a pair of sparsely inhabited rocks in waters between them, administered by Seoul as Dokdo, but claimed as Takeshima in Japan. The new manuals describe both sets of islands as "integral parts of Japanese territory" for the first time, the official said.
The manuals will also note that Takeshima is "illegally" occupied by South Korea, and that Japan does not even recognise the existence of a territorial dispute over the Senkaku islands, the official said.
The current manuals instruct teachers only to refer to a difference in Japanese and South Korean positions on Takeshima, while there was no remarks on the Senkakus.
"We called in the ambassador to lodge a strong protest ... after the Japanese education ministry maliciously included groundless allegations in textbook teaching manuals," South Korean Vice Foreign Minister Kim Kyou-Hyun told reporters who had been invited to witness the arrival of Japan's ambassador Koro Bessho for the dressing down. In a statement, the ministry accused Japan of "holding on to its past bad habit of distorting history and nostalgia over past imperialism".
The new manuals will be used for junior high school social studies and high school geography, history and civics classes, starting in April 2016. Although not mandatory, teachers are advised to follow their instructions when planning lessons.
Additional reporting by Associated Press