KUNASHIR, Kurile Islands (Reuters) - Russian President Dmitry Medvedev visited an island outpost seized by Russia from Japan at the end of World War Two stirring a diplomatic row with Tokyo, which demands their return.
Medvedev's visit to the island, one of four known as the Southern Kuriles in Russia and the Northern Territories in Japan, was likely to sour relations ahead of an Asia-Pacific leaders summit that Japan will host in mid-November.
It also spells more bad news from Japanese Prime Minister Naoto Kan, whose support ratings have sunk to around 40 percent after five months in office, partly due to his handling of a row with China over another set of islands that both nations claim.
"Japan's stance is that those four northern islands are part of our country's territory, so the president's visit is very regrettable," Kan told a parliamentary panel about Medvedev's visit, the first by a Russian leader, to the island called Kunashir in Russian and Kunashiri in Japanese.
Russian Foreign Minister Sergei Lavrov called the Japanese reaction "unacceptable."
"It is our land and the Russian president visited Russian land," Lavrov told a news conference.
The island chain, eight time zones from Moscow, stretches northeast from Japan's main northern island of Hokkaido to Russia's Kamchatka Peninsula. The island Medvedev visited lies some 10 miles from Hokkaido.
The dispute is a highly emotional issue in Japan and, among more nationalist circles, in Russia.
But unlike the dispute with China over islands in the East China Sea, which are near potentially vast maritime oil and gas reserves, this feud with Russia has more to do with the legacy of World War Two than hydrocarbon deposits.
The Soviet Union occupied the four islands at the end of the war and the territorial row has weighed on relations between Tokyo and Moscow ever since, precluding a formal peace treaty.
Politicians in both countries have used tough talk on the dispute to bolster their credentials as patriots. Analysts said the visit appeared intended to bolster Medvedev's image at home.
Medvedev, speaking to journalists on the island, promised more government investment. "It is important that there is development here, we will definitely be investing here."
"Life will be better here, like it is in central Russia."
State TV showed Medvedev tasting caviar on the islands at a fish plant and speaking to locals at shops.
Russia has a broader problem with the undeveloped areas of its far eastern regions, close to China and Japan. Their physical distance thousands of miles from Moscow has contributed to neglect of infrastructure and investment.
Medvedev's trip comes ahead of a November 13-14 Asia-Pacific Economic Cooperation (APEC) summit in Japan that he will attend with leaders of the 21-member group.
The islands are close to oil and gas production regions of Russia, but most people there live off fishing and Japan, a major fish consumer, would gain rich fishing grounds if the islands were returned.
Japan's dispute with China over the East China Sea islands is also decades-old, but flared in September when Japan detained a Chinese trawler captain whose boat collided with Japanese patrol ships near the isles, known as Senkaku and Diaoyu.
Hopes that relations between Asia's two biggest economies were on the mend were dashed on the weekend, when China abruptly canceled a planned meeting between Kan and Chinese Premier Wen Jiabao on the sidelines of an Association of Southeast Asian Nations (ASEAN) summit in Hanoi.
China became Japan's biggest trade partner last year, replacing the United States. Trade flows with Russia, however, are tiny by comparison.
Japan's exports to Russia totaled 306.5 billion yen ($3.8 billion) in 2009, about 2 percent of its exports to China and its imports from Russia came to 825.5 billion yen in 2009, accounting for 1.6 percent of Japan's total imports.
Kan's Democratic Party of Japan (DPJ) has been plagued in diplomatic disputes since sweeping to power last year.
Kan, who took office in June as Japan's fifth premier in three years, has seen his own popularity slide partly due to the perception that he mishandled the row with China.
In addition to row with China and now Russia, Japan has also seen ties with ally the United States frayed by a row over a U.S. airbase on southern Okinawa island, although concerns about China have helped relations with Washington improve recently.
"The Japanese government strained its ties with the United States over Futenma, then it mishandled the Senkaku issue with China," said political analyst Shigeki Hakamada, professor at Aoyama Gakuin University.
"If it were unable to take appropriate steps over this issue with Russia, the Kan government and the overall Democratic Party rule would suffer an extremely heavy blow."
(Editing by Ralph Boulton)