Japan today called on South Korea to end its "illegal occupation" of tiny islets in the strongest language so far in a dispute that has sent the two countries' relations to the lowest levels in years.
Lawmakers passed a symbolic resolution condemning South Korean President Lee Myung-bak's visit to the tiny, rocky outcroppings in the Sea of Japan earlier this month.
Critics of Prime Minister Yoshihiko Noda's government have called on him to take stronger action to protect those islands and others in the East China Sea that are controlled by Japan and also claimed by China and Taiwan.
Lawmakers also passed a resolution condemning China for its claims over those islands and accusing it of allowing activists to land on them.
In the resolutions adopted by Parliament, Japan called the visits by Mr Lee and the Chinese activists "extremely regrettable" and unacceptable.
Mr Noda said that Mr Lee had "illegally landed" on the islands called Takeshima in Japanese and Dokdo in Korean. Japan claims the islands and says South Korea "illegally" controls them.
Last week, a boatful of Chinese activists from Hong Kong landed on the islands known as Senkaku in Japan and Diaoyu in China. All 14 activists were arrested for illegal entry, released two days later and deported.
While the quick release of the Chinese activists by Japanese authorities kept the dispute with China from immediately flaring up, Japan's island row with South Korea has caused diplomatic relations between the two to plummet to their lowest levels in years.
Mr Noda sent Mr Lee a protest letter earlier this week, which Seoul rejected and sent right back. In the letter, Mr Noda also criticised Mr Lee for demanding Japan's Emperor Akihito apologise for the country's wartime actions.
Yesterday, a South Korean diplomat in Tokyo took the letter back to the Foreign Ministry but he was stopped by the security guard at the gate and could not get in. So the South Korean Embassy had the letter delivered by registered mail, which arrived today.
This time, Japan accepted it, apparently to keep the dispute from further escalating.
But Mr Noda said Japan has decided to take the island dispute to the International Court of Justice for a judgment. However, going to the international court requires consent from both parties.