South Korea's president on Friday made a surprise, election-year visit to islets also claimed by Japan, which quickly reasserted its sovereignty there, recalled its ambassador from Seoul and warned that the trip would worsen the countries' strained relations.

President Lee Myung-bak's trip to the tiny, rocky outcroppings between the countries was the first by a South Korean president, officials in his office said. It came as his popularity is dropping and his conservative party jockeys for votes ahead of a presidential election in December. Lee is nearing the end of his single, five-year term and cannot run for re-election.

A set of remote islands called Dokdo in Korean and Takeshima in Japanese is seen in this picture taken from a helicopter carrying South Korean President Lee Myung-bak. (Reuters)

Japanese Prime Minister Yoshihiko Noda told reporters the islets are "our sovereign territory.

"This is completely unacceptable," Noda said. "It is deeply regrettable." He said Japan's ambassador to Seoul was being called back to Tokyo. Japanese officials also called Seoul's representative in Tokyo to the Foreign Ministry to hear Japan's complaints.

Lee's visit came on the eve of the men's bronze medal Olympic soccer match between Japan and South Korea and ahead of South Korea's commemoration Wednesday of the peninsula's independence in 1945 from 35 years of Japanese colonial rule.

South Korean police stationed on islets

South Korea stations a small contingent of police officers on the disputed islets in a show of control, but Japan maintains that the rocks are its territory. Tokyo renewed the claim last month in an annual defence report.

Lee placed his hand on a rock carving that says "South Korean territory" during the visit to the islets — called Dokdo in Korean and Takeshima in Japanese. He also told police officers there that the islets are "worth sacrificing lives for," according to his office.

Japanese Foreign Minister Koichiro Gemba strongly protested. "It is incomprehensible why he would make this trip at this time," Genba said.

'This is completely unacceptable. It is deeply regrettable.'—Japanese Prime Minister Yoshihiko Noda

The largely uninhabited islets surrounded by fish-rich waters have long been a source of discord, even though Japan and South Korea are strong U.S. allies, share vibrant trade and tourism ties and are partners in diplomatic efforts to persuade North Korea to abandon its long-range missile and nuclear arms programs.

History and territorial disputes, however, trouble the relationship. Many people on the Korean Peninsula harbour deep resentment stemming from Japan's brutal colonization.

Korean used as WW II sex slaves

South Korea and Japan also remain at odds over what many South Koreans say is Japan's failure to properly address its past actions, including its Second World War-era use of Korean women as sexual slaves for its soldiers.

South Korea controls the islands with a coast guard presence and plans to beef up maritime research. (Reuters)

In late June, after a political outcry in South Korea, Seoul and Tokyo put on hold an intelligence sharing pact that had been seen as a breakthrough in their relations.

South Korean activists last year placed a statue of a girl representing victims of Japanese sexual slavery in front of the Japanese Embassy in Seoul. Japanese officials have apologized, but Tokyo has refused repeated demands from individuals for reparations, saying the matter was solved by international peace treaties.

U.S. State Department spokesman Patrick Ventrell expressed hope for good relations between the two key U.S. allies after being asked about reports that Lee was planning to visit the disputed islets.

Last year, Seoul banned three conservative Japanese lawmakers from entering South Korea after they arrived at a Seoul airport with announced plans to travel near the islets.