Japan's Foreign Ministry on Sunday protested South Korea's maritime survey near the Sea of Japan islets of Takeshima, claimed by Japan but controlled by South Korea.
According to the ministry, a South Korean survey ship was spotted on the day dropping apparent wires into waters within Japan's exclusive economic zone southwest of the islets.
Hiroyuki Namazu, director-general of the ministry's Asian and Oceanian Affairs Bureau, lodged a protest by telephone the same day with a senior official at the South Korean Embassy in Tokyo, arguing that the survey was unacceptable and should be stopped immediately.
It was the fourth time this year that Japan has protested a South Korean maritime survey near Takeshima.
The South Korean side gave no prior notice of the latest survey, according to the Japanese ministry.