Russian President Dmitry Medvedev is set to arrive on one of the disputed Kuril Islands Monday, news reports said.
Medvedev is leaving for Kunashiri Island on his way back from his visit to Vietnam, an unnamed source close to the Russian presidential office was quoted by Kyodo News as saying.
Japan, which also claims the Russian-held islands, has warned his visit would seriously hurt the two countries' relations, DPA reported.
Medvedev would be the first Russian leader, including those of the Soviet Union, to visit the islands, which Japan refers to as the Northern Territories.
The Russian president planned to visit the islands in September, but bad weather prevented him from doing so.
The islands were occupied by Soviet forces during the closing days of World War II. The two countries have not signed a peace treaty because of the row over the islands.