The war games on Etorofu and Kunashiri islands in the Sea of Okhotsk -- part of an island chain including Shikotan and Habomai islands, which Tokyo calls the Northern Territories and Russia the Kuril Islands -- were called "totally unacceptable" by Japanese Prime Minister Shinzo Abe, Japanese broadcaster NHK reported Thursday.
The Soviet Union seized the islands after World War II and their inhabitants returned to Japan. Although Japan and the Soviet Union never formally ended their hostilities, a 1956 agreement included a promise to resolve the dispute over the islands.
The drills began Tuesday and brought 1,000 Russian troops and 100 "units of combat and special equipment" to the site, the Russian news agency RIA Novosti said. The tiny islands are sparsely populated and contain only the debris of Soviet-era military equipment, but the head of Russia's Eastern Military District, Col. Gen. Sergei Suvorkin, told RIA Novosti in June plans are underway to construct permanent military facilities on the islands by 2016.