Japan reacted quickly and angrily to the Russian president's visit to a disputed island in the Kuril chain. The territorial dispute has overshadowed Russo-Japanese relations since the end of World War II.
As Russian President Dmitry Medvedev on Monday made a brief tour of Kunashir - or Kunashiri to the Japanese - government officials in Japan wasted no time expressing strong displeasure.
Speaking at a parliamentary committee session, Prime Minister Naoto Kan reiterated that the Russian-held islands are Japanese territory.
Kan says considering that Japan maintains its position that the four northern islands are Japanese, Monday's visit by the Russian president is deplorable.
Moscow's ambassador to Tokyo was quickly summoned to the Foreign Ministry to receive a diplomatic protest.
Ambassador Mikhail Bely, speaking to reporters at the ministry, did not have conciliatory words, however.
The Russian ambassador says he told the Japanese Foreign minister the visit should be considered a Russian domestic issue and Japan should deal with it calmly and in a balanced manner.
Russian media report that Mr. Medvedev, during his few hours on Kunashir, toured a geothermal power plant, a seafood processing facility and a new kindergarten. The reports say the president promised to invest funds for development of the remote region.
The Kuril islands lie just north of Japan's main northern island, Hokkaido. The 1,300-kilometer-long archipelago has been under Moscow's control since Soviet troops seized it in the closing days of World War Two. Japan claims the four southernmost islands as its own.
The matter has remained unresolved since 1945, so the two countries have yet to sign a peace treaty to formally end the war, which prevents closer political and economic ties between the neighbors.
Over the years there have been discussions between Moscow and Tokyo on Russia returning two of the four islands.
The intensification of the dispute with Moscow is the latest diplomatic setback for Prime Minister Kan, in office for less than five months. His government recently found itself involved in one of the worst spats in years between Tokyo and Beijing.
That disagreement also involves disputed small islands. It flared up following a collision between a Chinese fishing vessel and a Japanese Coast Guard patrol ship in the East China Sea.