Russia is demanding that Japan investigate a protest that took place outside the Russian embassy in Tokyo, and warning that it could open its own criminal case in the matter.
Last month's demonstration desecrated a Russian flag to protest Russia's holding of four Pacific islands it seized from Japan in the waning days of World War II.
The islands _ which Russia calls the Kurils and Japan calls the Northern Territories _ have been a consistent irritant between Moscow and Tokyo. But tensions rose sharply after Dmitry Medvedev in November became the first Russian president to visit the islands.
The Russian Foreign Ministry called in the Japanese charge d'affaires on Wednesday to demand the investigation of the protesters, "establishment of their guilt and proper punishment. If this is not done, the Russian authorities will have to take this circumstance into account in future steps regarding Japan," a ministry statement said.
In a separate statement, ministry spokesman Alexander Lukashevich said Russia "reserves the right to call Japanese hooligans to criminal responsibility in accordance with Russian law."
The islands give Russia a military toehold just six miles (10 kilometers) off the northeastern tip of Japan's northern Hokkaido Island. The islands are surrounded by rich fishing grounds and are believed to have offshore oil and natural gas reserves, plus gold and silver deposits.
Since Medvedev's visit, Russian officials have vowed that weapons for the troops on the island will be upgraded. News reports also have cited defense sources as saying that could include sending new S-400 air-defense rockets to the islands and that some of the Mistral assault ships that Russia has contracted to buy from France would be deployed in the Pacific Fleet, as part of efforts to protect the Kurils.
Foreign Minister Sergey Lavrov also has angered Japan by saying Russia will pump more money into the Kurils and invite investors from other nations, including South Korea and China, to follow suit.