Japan lodged a protest over what it said was a South Korean research vessel sailing in waters near disputed remote islets in the East Sea, also known as the Sea of Japan.
Masaaki Kanai, head of the Japanese Foreign Ministry's Asian and Oceanian Affairs Bureau, spoke to South Korean diplomats on Friday after the detection of the ship, according to a statement from the Japanese Foreign Ministry on Saturday.
The vessel, named ONNURI, dropped something "resembling a wire into the sea," according to the statement.
"Takeshima is clearly an integral part of Japanese territory, in light of historical facts and under international law," it added.
The South Korean side has not yet commented on the Japanese Foreign Ministry statement.
The islets, called Takeshima in Japan and Dokdo in South Korea, are controlled by Seoul but claimed by Tokyo.
