Seoul:
South Korea on Thursday returned a protest letter from Japan's prime minister without answering it, further angering Tokyo amid a bitter row over disputed islands.
Prime Minister Yoshihiko Noda's letter was returned because it could be used by Tokyo to strengthen its territorial claim over the islands in the Sea of Japan (East Sea), Seoul's foreign ministry said.
The letter, addressed to South Korean President Lee Myung-Bak had been kept at the South's embassy in Japan. It was returned via registered mail as Tokyo refused to receive it in person, the ministry said.
In Tokyo, Chief Cabinet Secretary Osamu Fujimura denounced the decision.
"Under usual protocol, it is inconceivable that letters exchanged between leaders are sent back," Mr Fujimura, the government's top spokesman, told a news conference.
"I hope (Lee) will accept the letter, which was sent to deliver our prime minister's thoughts."
Relations have sharply worsened since Mr Lee paid a surprise visit on August 10 to the Seoul-controlled islands known as Dokdo in Korean and Takeshima in Japan.
He said his trip, the first by a South Korean president, was intended to press Japan to settle grievances left over from its colonial rule over Korea from 1910-45.
Mr Lee further angered Japan by saying later that Emperor Akihito must sincerely apologise for past excesses should he wish to visit South Korea.
Mr Noda in his letter expressed regret at the islands visit and at Mr Lee's remarks on Akihito, Yonhap news agency said.
The visit infuriated Japan, which recalled its ambassador from Seoul. Tokyo also cancelled a finance ministers' meeting scheduled for this month and said it would review a foreign exchange swap agreement with Seoul.
South Korea has rejected Japan's proposal that the two countries ask the International Court of Justice to settle the dispute.
The dispute heated up again after Japanese Foreign Minister Koichiro Gemba on Wednesday described Seoul's control over the islands as an "illegal occupation".
"We lodge a strong protest... and demand Japan's foreign minister immediately withdraw his remarks," said foreign ministry spokesman Cho Tai-Young.
About 400 South Korean activists waving national flags rallied outside the Japanese embassy in Seoul, urging Tokyo to withdraw its claim to the islands.
They also demanded Tokyo compensate elderly Korean women forced into sexual slavery for Japanese troops in World War II and called on Akihito to make a sincere apology.
The embassy was guarded by about 100 riot police with plastic shields but there was no violence.
Seoul accused Tokyo of neglecting diplomatic etiquette by disclosing the contents of Noda's letter in advance.
But one analyst said South Korea should have replied to Mr Noda's letter because Japan sees Seoul's reaction as rude.
"There was no need to return the letter as it could worsen the emotional atmosphere between the two parties," Lee Myon-Woo, a researcher at South Korea's Sejong Institute think-tank, told AFP.