A delegation led by Deputy Prime Minister Sergei Ivanov visited Kunashir and Iturup, part of the Kuril Island chain, Sunday during a tour of the Russian Far East. The Japanese Foreign Ministry submitted a formal complaint to the Russian Embassy in Tokyo Monday.
Japan still claims the islands, seized by the Soviet Union, of which Russia was a part, in the final days of the war in 1945. The dispute has prevented the signing of a peace treaty for 66 years.
"We remind you that Russia's sovereignty over these islands has indisputable international and legal foundations," the Russian Foreign Ministry said in statement reported by RIA Novosti. "So 'regrets' regarding the Russian officials' visit to the islands are inappropriate."
When Russian President Dmitry Medvedev visited one of the islands last November, Japanese Prime Minister Naoto Kan called the trip "inexcusable rudeness," bringing an angry reaction from the Kremlin.