Japan has recalled its ambassador to Moscow after protests greeted the visit of Russian President Dimitry Medvedev to the disputed island of Kunashir on Monday.
The island is one of four close to Japan that are claimed by both Moscow and Tokyo.
The incident has caused a headache for the Japanese government ahead of a major Asia-Pacific leaders’ summit taking place in the country in a few weeks.
Chief Cabinet Secretary Yoshito Sengoku said they would have to “think again about what measures would be effective against this sort of action.”
He went on to say that he does not believe “there is only one solution to the problem.”
Sovereignty of the islands, located between Japan and Russia, has been disputed since Soviet troops moved in at the end of the Second World War.
The dispute has prevented the signing of a formal peace treaty between the two countries.
Japan’s Prime Minister Naoto Kan said the Russian president’s visit was “regrettable”
The islands are of interest to both countries not least because of their abundant fish, oil and gas reserves.