South Korea announced that it would return a letter from Japanese Prime Minister Yoshihiko Noda, in a move reflecting growing diplomatic tensions between the two countries, The Nikkei reports.

Prime Minister Noda wrote to Mr. Lee over his unprecedented visit to the Dokdo Islands, known as Takeshima in Japan, and said that Japan plans to take the territorial dispute to the International Court of Justice.

Noda also described Mr. Lee’s remarks, asking Emperor Akihito to apologize for Japan’s past colonial rule of the Korean Peninsula, as “regrettable”. The letter urged South Korea to act accordingly and avoid damage to Japan-South Korea ties.

On Wednesday, a senior South Korean foreign ministry official confirmed the decision to return the letter to Noda. This comes as Japan switches ambassadors on its diplomatic front line.