Russian President Dmitry Medvedev has arrived in the Kurils, defying Japan's warnings not to visit the disputed islands in the Pacific Ocean.
Japanese Prime Minister Naoto Kan immediately described Mr Medvedev's visit to Kunashir Island as "regrettable".
Earlier, Russia's top diplomat Sergei Lavrov warned that Mr Medvedev was free to visit any Russian region he chose.
The islands have been under Moscow's control since the end of World War II.
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More News On Russian President Medvedev's Visit To The Kuril Islands
Medvedev visits disputed islands -- Al Jazeera
Japan upset as Russian leader visits disputed isle -- Reuters
Russian president's visit to disputed isle "hurts Japanese public sentiment": Japanese FM -- Xinhuanet
Japan to summon Russian ambassador on isle row-Jiji -- Reuters
Japan says to monitor Russian comments on isle row -- Reuters
Russian President Visits Disputed Island -- Wall Street Journal
Visit to Kurils 'regrettable' -- Straits Times