Foreign Minister Seiji Maehara - who left for Moscow on Thursday as Russia has taken an increasingly assertive stance on the dispute - said Japan stands by its claim to the islands in the southern Kuril chain that were occupied in the closing days of World War II by the Soviet army. The disagreement has kept the two nations from signing a peace treaty to formally end those hostilities.
"Whether Russia sends its leaders there or boosts its defenses, it still has no claim under international law to occupy the islands," he said.
President Dmitry Medvedev traveled to the island's last year - becoming the first Russian leader to do so. He has stressed that he sees the islands as his country's territory.
Medvedev announced Wednesday that Russia will send more weapons to its islands in north Pacific. He said his country must strengthen its presence there and that the new weapons will protect the islands as an "inalienable part of Russia."
The Russian state news agency ITAR-Tass cited a Defense Ministry source as saying that some of the four Mistral assault ships would be deployed in the Pacific Fleet, in part to defend the southern Kurils.
Japanese Chief Cabinet Secretary Yukio Edano responded Thursday by saying that Japan is closely watching Russian military activity in the region.
Japanese Prime Minister Naoto Kan recently called Medvedev's visit "an outrage."
Maehara, who on Thursday called the territory an integral part of Japan, was expected to express similar concern during his visit to Moscow. He said before leaving that he wanted to agree on specific steps toward ending the dispute and finally signing a peace treaty with Russia.
Japan has said that the door to economic cooperation is not completely closed, but progress must also be made on the island dispute.
"There is room for expanding our cooperation in such fields as economy and energy," said Noriyuki Shikata, a spokesman for the prime minister. "In order to expand such cooperation, it is essential from our viewpoint that we will overcome the territorial issues."
Associated Press writer Mari Yamaguchi contributed to this report.