Japanese Defense Minister Itsunori Onodera said on Tuesday Tokyo had asked Russia to reduce its military activity on a disputed island chain in the Pacific after Moscow beefed up its forces there in response to what it sees as a potential threat.
The territorial dispute over the islands, known as the Kuriles in Russia and the Northern Territories in Japan, is so acrimonious that Moscow and Tokyo have not yet signed a peace treaty to mark the end of World War II.
Prime Minister Dmitry Medvedev approved the deployment of Russian warplanes on one of the disputed islands in February, accelerating the area's militarization at a time when Moscow's ties with Tokyo are strained over the roll-out of the Aegis U.S. missile system.
Moscow has also deployed its newest missile defense systems to the islands and plans to build a naval base there even as it continues talks about the territorial dispute.
"We have asked the Russian side to take particular measures because Russia is building up its military potential on the four northern islands," Onodera said after meeting his Russian counterpart, Sergei Shoigu, in Moscow.
The Soviet Union seized the islands from Japan at the end of World War II.
Onodera said that the ground-based Aegis ballistic missile defense stations were solely intended to defend Japan and did not pose any threat to Russia.
Russia is concerned Japan is allowing Washington to use its territory as a base for a U.S. military build-up in north Asia under the pretext of countering North Korea.
President Vladimir Putin is expected to meet Japanese Prime Minister Shinzo Abe in September in Vladivostok in Russia's Far East.