South Korean President Lee Myung-Bak is planning to visit on Friday a group of islets in the Sea of Japan (East Sea) controlled by Seoul but claimed by Tokyo, Japanese media said.
Seoul notified Tokyo on Thursday of the trip to the islands, called Dokdo in South Korea and Takeshima in Japan, Kyodo News and other Japanese media reported, quoting diplomatic sources.
It would be the first visit to the islands, roughly half way between the two countries, by a South Korean head of state since the row between Seoul and its former colonial ruler began decades ago.
Japanese Foreign Minister Gemba told reporters Friday morning the planned trip would have consequences.
"If the visit is realised, it would have a great impact on Japan-South Korea relations," he said, according to Jiji Press. "This contradicts the Japanese position and we have to respond firmly."
Jiji said Lee was expected to leave Seoul by helicopter later Friday morning and would reach the island in the afternoon for a brief stop before heading home. The agency said he would be accompanied by two of his ministers.
The reports come just days before the August 15 anniversary of Japan's World War II surrender and as South Korea gears up for a presidential poll later in the year.