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 had said 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. The islands are known in Russia as the Southern Kurils. Japan calls them the Northern Territories. On Monday, Mr Medvedev became the first Russian leader to set foot on the Kuril Islands. The territorial dispute has strained relations between Tokyo and Moscow ever since World War II, preventing the signing of a formal peace treaty.