"It's an unusual action in terms of diplomatic protocol," Japanese Chief Cabinet Secretary Osamu Fujimura commented on the decision made by South Korea of returning Japanese Prime Minister Yoshihiko Noda's letter on Thursday.

Fujimura's remarks came after a senior South Korean foreign ministry official said Wednesday that Seoul will return a diplomatic letter from Tokyo to protest South Korean president Lee Myung-bak's visit to the disputed islands and remarks made by Lee regarding Japan's emperor.

"We have not yet received any message of returning the letter from South Korea officially. I think it will not happen," Fujimura told a press conference.

In the letter, sent last Friday, Noda said Japan will propose to resolve the issue peacefully under the international law and urged Seoul to act with a broad view of bilateral relations.

Japan has sent its ambassador to South Korea back to Seoul on Wednesday after temporarily recalling him in protest against Lee' s visit to the South Korean-controlled disputed isles in the Sea of Japan which Japan claims are part of its territory.