Japan has lodged a protest with South Korea for its operation of a survey ship within Japanese territorial waters around a group of Sea of Japan islets disputed by the two countries, the top government spokesman said Friday.

Japan demanded twice that South Korea explain the ship's activities around the Tokyo-claimed, Seoul-controlled islets after it entered the waters on both Wednesday and Thursday, Chief Cabinet Secretary Yoshihide Suga told a news conference.

The long-standing dispute over the rocky islets, called Takeshima in Japanese and Dokdo in Korean, has been a major source of diplomatic tension between the two neighbors.

"Survey activity without the agreement of Japan is unacceptable," Suga said, adding that the vessel moved out of the waters after warnings by the Japan Coast Guard.

South Korea conducted a similar maritime survey in July 2006, drawing protest from Japan at the time.

Japanese diplomatic experts believe the South Korean action was aimed at strengthening control of the islets, rather than purely investigating fishery resources in the area.