Russia and Japan are involved in a diplomatic row over Russian President Dmitry Medvedev's visit to an island also claimed by Japan.
Medvedev flew into Kunashir Island for a four-hour visit today, the first by a Russian president to one of four Kurile Islands also claimed by Japan.
The Soviet Army occupied the Kurile Islands at the end of World War II, but Tokyo claims the southernmost four as Japanese territory. The territorial dispute has weighed on relations between Tokyo and Moscow, preventing the signing of a peace treaty.
Japanese Prime Minister Naoto Kan called Medvedev's visit "very regrettable."
Japan's foreign minister summoned the Russian ambassador to Tokyo over the visit.
Russian Foreign Minister Sergei Lavrov said Japan's reaction is "unacceptable."
"The reaction of the Japanese side to President Medvedev's visit to the Kuriles was unacceptable," Lavrov said. "It is our land and the Russian president visited Russian land."
Lavrov said the Japanese ambassador to Moscow would be summoned to be told Russia's position.
compiled from agency reports