Video | Russian isle visit sparks diplomatic row with Japan

The row comes at a bad time for Japan, as it finds itself in the midst of struggling to mend ties with rising rival economic super-power China.

Medvedev's visit to the island, one of four known as the Southern Kuriles in Russia and the Northern Territories in Japan, is likely to sour diplomatic relations just before an Asia-Pacific leaders summit that Japan will host in mid-November.

Japanese Prime Minister Naoto Kan swiftly expressed regret over the visit -- the first by a Russian leader.

"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.

Japanese Foreign Minister Seiji Maehara was also reported as saying that any such visit would "hurt the feelings of the Japanese people." Just last month, he affirmed how a visit by Medvedev would severely harm relations.

However, Russian Foreign Minister Sergei Lavrov denied that Medvedev's trip was politically charged. "The territory of the Russian Federation is the territory of the Russian Federation," Lavrov said.

It is speculated however that the visit appeared intended to bolster Medvedev's image at home.