Japan's Foreign Ministry lodged a "strong protest" Wednesday over Russian military exercises on two islands seized by the Soviet Union after World War II and claimed by both countries.
Japanese Prime Minister Shinzo Abe called the war games on Etorofu and Kunashiri islands "totally unacceptable," the NHK broadcaster reported from Tokyo.
The two islands, as well as Shikotan and Habomai, were seized by Soviet troops after Japan surrendered in 1945 and their Japanese inhabitants were deported.
Russia and Japan never signed a peace treaty to formally end their wartime hostilities, although an agreement restoring diplomatic relations in 1956 included a commitment by the two Pacific neighbors to work toward resolving the islands dispute.
Tokyo considers the four islands part of its Northern Territories extending from Hokkaido, while Russia says they are part of the Kurils chain south of Sakhalin.
Russia's RIA Novosti news agency said the drills, which began on Tuesday, brought more than 1,000 troops, five attack helicopters and 100 "units of combat and special equipment" to the islands.
Abe has met with Russian President Vladimir Putin five times since returning to the prime minister's office in 2012 in an apparent effort to improve relations with Japan's northern neighbor. He broke ranks with most leaders of the industrialized world by attending the opening ceremonies of the Olympic Winter Games in Russia's Black Sea port of Sochi in February in spite of concerns about Moscow's human rights record.
But Abe in June joined other Group of Seven democracies in imposing sanctions on Moscow for its annexation of Ukraine's Crimea territory and its role in the deadly insurgency in eastern Ukraine.
In June, the head of Russia's Eastern Military District, Col. Gen. Sergei Suvorkin, told RIA Novosti that the Kremlin had plans to build 150 military facilities on the disputed islands by 2016.
Japan is also party to disputes with other Asian neighbors over the sovereignty of islands in the East and South China seas.Follow @cjwilliamslat for the latest international news 24/7