The remains were discovered during a survey this month by a joint team from the two countries, the official Xinhua news agency said, as their relationship grows increasingly intertwined.
The preliminary search, in which six experts from Russia participated, was for the remains of 413 Soviet soldiers who died fighting Japanese forces on Huoshan mountain and in three nearby villages in the northeastern province of Heilongjiang, Xinhua said.
According to previous reports it was their first such joint search.
Beijing and Moscow, allies and then adversaries during the
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Cold War, have over the past quarter century found common ground internationally and often take similar stands at the UN Security Council where they have permanent veto powers.
They have also developed increasingly closer economic ties
as China eyes the vast hydrocarbon resources of Russia, which is seeking stable markets amid Western sanctions over its annexation of Crimea and fighting in eastern Ukraine.
As well as the US dropping atomic bombs on the Japanese cities of Hiroshima and Nagasaki on August 6 and August 9, the Soviet entry into the conflict is seen as a major factor hastening Japan's suing for peace.
Both China and Russia continue to have fraught relations with Japan.
Beijing frequently criticises Tokyo over its attitude towards the war and is at odds with it over a territorial dispute in the East China Sea.
Both Russia and China are planning major commemorations of the war's end.
Chinese troops will participate in a parade on May 9 in Moscow to mark the 70th anniversary of victory in World War II, though Western leaders are largely staying away over Russia's role in the Ukraine conflict.
Beijing, meanwhile, has announced that it will hold a parade later this year to celebrate victory over Japan as well as the broader global defeat of fascism.