Tokyo filed a protest with Seoul on Tuesday over an alleged intrusion into its exclusive economic zone (EEZ) by a South Korean research vessel.
Japan's Foreign Ministry said in a statement that the survey ship, the NARA, had dropped a wire-like object into the sea, Kyodo News reported Wednesday.
The incident happened near disputed islets called Takeshima in Japan and Dokdo in South Korea, controlled by Seoul but claimed by Tokyo. The remote islets are located in the East Sea, also known as the Sea of Japan.
Masaaki Kanai, head of the ministry's Asian and Oceanian Affairs Bureau, told a senior official of the South Korean Embassy in Tokyo that Japan cannot accept a survey being done in its EEZ without its consent.
A senior Japanese Embassy official also lodged a protest with the South Korean Foreign Ministry in Seoul.
The South Korean side has not yet commented on the Japanese statement.
This development followed a similar incident on Aug. 15 when a different South Korean maritime research vessel, the ONNURI, was spotted engaging in a suspected survey west of the islets.
