South Korean President Lee Myung-Bak has angered Japanese authorities by a planned visit to islands at the core of a territorial dispute with Japan.
The visit on Friday would be the first such trip by a South Korean leader to the islands, known as Takeshima in Japan or Dokdo in South Korea.
South Korea controls the islands with a coast guard presence and plans to enhance maritime research in the area also rich in marine life.
Japan swiftly issued a terse warning, urging the cancellation of the trip.
"The visit would definitely have a large impact on relations between the two countries, already tense due to the dispute over the islets,” Japanese Foreign Minister Koichiro Gemba said on Friday
Despite the proximity of the two East Asian nations, bilateral relationships remain dogged by the dispute.
Officials in South Korea said the visit would highlight the islands' importance as a natural reserve and was not aimed at stirring up trouble.
The Islands are said to contain large deposits of natural gas and oil.