SEOUL - South Korean President Lee Myung Bak yesterday made a surprise visit to islets at the centre of a long-running territorial dispute with Japan, ignoring warnings from Tokyo that it would worsen the neighbours' already strained relations.

His trip to the rocky, largely uninhabited outcroppings in fish-rich waters between the countries is the first by a South Korean president, officials said. He also briefly visited nearby Ulleung Island, which is not part of the territorial dispute.

The trip to the islets - called Dokdo in Korean and Takeshima in Japanese - comes as Mr Lee's conservative party jockeys for votes ahead of December presidential elections.

Mr Lee, whose popularity has steadily dropped, is at the end of his five-year term and cannot run for re-election.

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

The visit came on the eve of the men's bronze medal Olympic football match between the two nations, and ahead of South Korea's commemoration next Wednesday of the peninsula's independence in 1945 from 35 years of Japanese colonial rule.

South Korea has a small police contingent on the islets in a show of control, but Japan says the rocks are its territory. It renewed the claim last month in an annual defence report.

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

"This is completely unacceptable," he 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.

Japanese Foreign Minister Koichiro Gemba upbraided his South Korean counterpart by phone and said the visit "would have a major negative impact on our people's sentiment".

"Our side has no choice but to take proper measures in response," he said.

The outcroppings have long been a source of discord, even though the two countries are both United States 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 programmes.

One analyst said Mr Lee's trip was an over-reaction to diplomatic strains and should have been considered more thoroughly.

Strategically, the visit to Dokdo would be one of the strongest actions the President could take, said Mr Jin Chang Soo of South Korea's Sejong Institute think-tank.

"In the long term, considering there will be many problems (between the two countries), I doubt whether this is the right time to play this card," he told Agence France-Presse.

He said Japan was currently unstable, engaged in territorial disputes with other nations, "and we've just added fuel to the fire. What good can it do?"

ASSOCIATED PRESS, AGENCE FRANCE-PRESSE