TOKYO -- The Japanese government has demanded an explanation from Seoul after a South Korean marine survey ship entered waters near a disputed set of islets in the Japan Sea over the weekend.
The features, which are currently under Seoul's control, are referred to as Takeshima by Japan and Dokdo by South Korea.
"We have strongly protested that South Korea is not allowed to conduct marine survey activities without the consent of Japan," Chief Cabinet Secretary Yoshihide Suga told reporters after a Cabinet meeting on Tuesday.
Tokyo plans to send the Cabinet Office's parliamentary secretary to a ceremony marking "Takeshima Day" on Friday, which will be held in the town of Matsue, on the Japan Sea coast.
The vessel's activity may have been part of an attempt by the South Korean authorities to demonstrate their effective control over the islets in the run-up to the ceremony.
According to Suga, the survey ship entered Japanese waters once on Feb. 15 and another time from Feb. 17 to 18.
"Japan Coast Guard patrol vessels remain vigilant near the waters," he said, adding that the ship had now re-entered South Korean waters by crossing the median line between the two countries.
Recent reports have claimed that South Korea a few years ago conducted a survey of the seafloor in the vicinity of the islets without Japan's consent. According to the reports, this involved the collection of mud samples in Japanese territorial waters and their contiguous zone.
"We are trying to establish facts and details," Suga said when asked about the reports.
Mitsuhiro Miyakoshi, Japan's minister of state in charge of territorial issues, said the islets were "clearly an integral part of Japan's sovereign territory, both in light of historic facts and in terms of international law," at a news conference Tuesday.
"Survey activity conducted without our country's consent will not be tolerated. It's very regrettable," he added.
"We will step up efforts to send [our message] in Japan and to the international community," Miyakoshi said, "to ensure our position regarding the Takeshima issue is understood correctly at home and abroad, maintaining consistency with our diplomatic policy."