SEOUL - South Korea dispatched a ship to survey waters around a group of islands it contests with Japan despite repeated warnings from the Japanese not to do so, a news report said.
The boat left the South Korean port of Busan with 20 crew members on board at 10:30 pm (1330 GMT), South Korea's Yonhap news agency said, quoting the National Oceanographic Research Institute, which organised the survey.
Tension between the two countries first mounted over the issue in April when Japan infuriated South Korea by planning its own survey of the islands, known as Takeshima in Japanese and Dokdo in Korean.
South Korea, which remains bitter over Japan's brutal colonial rule from 1910 to 1945, has been incensed by Japanese claims to the islands in the Sea of Japan, or East Sea.
Tokyo said it needed to conduct its study in April so it could submit a counter-proposal to an international oceanographic meeting where Seoul planned to propose Korean names for features on the seabed.
The two countries defused the row after South Korea retracted the proposal and Japan cancelled its survey.
But it resurfaced on Thursday when Seoul announced it was planning to conduct a survey from Monday until July 14.
Japan's coast guard chief warned it would take "appropriate actions" if South Korea went ahead with the survey, although he ruled out seizing the 2,500-tonne ship.
"If South Korea conducts the survey without notifying us, the coast guard will take appropriate actions on the site based on international and domestic laws while abiding by international laws," Japan Coast Guard Director General Hiroki Ishikawa said on Friday.
Japan's Chief Cabinet Secretary Shinzo Abe also urged South Korea to rethink the plan, saying Seoul had not notified Japan of the survey.