Tokyo, Japan (CNN) -- Japan's foreign minister has warned that a possible trip by Russia's president to islands both countries claim could harm diplomatic relations, Japanese media reported.
The disputed, sparsely populated islands are known as the Northern Territories in Japan and the Southern Kurils in Russia, according to Japan's Kyodo news agency.
Earlier Wednesday, Russian President Dmitry Medvedev said he would travel to the islands soon, Russian state media reported.
"I will fly there for sure. This is a very important region in our country," Medvedev said, according to the state-run ITAR-TASS news agency.
Such a trip would "severely hurt bilateral ties," Japanese Foreign Minister Seiji Maehara told Kyodo.
Maehara said he had asked Russia's ambassador to Japan to convey his concerns to Moscow.
Kyodo reported that the islands were seized by Russia in the final days of World War II, and the continued dispute over them has prevented the two countries from signing a post-World War II peace treaty. The agency said no Russian leaders have visited the islands.
Another territory dispute involving Japan sent shock waves through diplomatic circles this month, after Japanese authorities detained a Chinese fishing boat captain.
Zhan Qixiong was detained earlier this month after his trawler collided with Japanese vessels near disputed islands in the East China Sea.
The arrest has increased tensions between China and Japan, with Beijing accusing Japan of "unlawful and invalid" arrest of the captain and his crew of 14, according to the Chinese state-run Xinhua new agency.
The captain and his crew were later released by Japan.