Japan says it will strongly protest to Russia over military exercises it is staging on disputed islands off northern Japan.
The prime minister, Shinzo Abe, said the drills were totally unacceptable.
He has been trying to improve relations with Russia at a time of high tension with China over other islands, but has angered Moscow by backing sanctions over the crisis in Ukraine.
Russian media said 1,000 troops and attack helicopters were taking part.
The dispute over the islands, known as the Southern Kuriles in Russia and the Northern Territories in Japan, have prevented the two countries signing a formal peace treaty.
They were seized by Soviet troops at the end of the Second World War and the Japanese population was expelled in the years that followed.
Mr Abe met the Russian president, Vladimir Putin, five times during his first year in office.
He has been trying to improve relations with a country that is becoming an ever larger source of oil and gas to the region.
Japan, as a close ally of the United States, imposed sanctions on Moscow over the annexation of Crimea, but has kept them lighter than some other countries for fear of antagonising Mr Putin.