300,000 soldiers, 36,000 military vehicles, 1,000 planes and 80 warships will take part in the drills.
The weeklong exercises reprise massive Soviet war games that took place at the height of the Cold War.
Up to 3,500 Chinese military personnel will take part in the "main scenario" of the drills at the Tsugol proving ground, Defense Minister Sergei Shoigu said on September 4. Mongolia also sent a small contingent.
Some experts see the war games as a message to Washington from Moscow and Beijing.
Russian President Vladimir Putin is expected to attend the games after hosting an economic forum in Russia's far eastern city Vladivostok where his Chinese counterpart Xi Jinping is one of the prominent guests.
The Chinese media touted the Chinese involvement in the maneuvers as the country's largest-ever dispatch of forces overseas for war games.
Xi said the two countries' friendship was "getting stronger all the time".
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"China, by sending a PLA element to train with the Russians, is signalling that USA pressure is pushing it towards much closer military cooperation with Moscow".
"This is a milestone for Sino-Russian military ties", Alexander Gabuev, a senior fellow at the Carnegie Moscow Center, wrote on Twitter.
"[It also] signifies that the political strategic trust and military cooperation between China and Russian Federation has reached a historic high".
The most intense exercises will be held in the Tsugol range, about 90 miles from the Chinese and Mongolian borders.
"The focus of the drill has expanded from anti-terrorism to allied defense and counterattack", a China defense ministry statement said.
North Atlantic Treaty Organisation has denounced the exercises as a rehearsal for a "large-scale military conflict".
"The fact that the drill is vastly greater in size than previous exercises in the same location is also part of the natural progression of Russia's practice for war", Giles said.
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In this photo provided by the Russian Defence Ministry Press Service on Tuesday, Sept. 11, 2018, a Russian soldier guards an area during the military exercises in the Chita region, Eastern Siberia, during the Vostok 2018 exercises.
The Kremlin has insisted the exercises are routine and purely defensive.
Russian Federation has started its biggest "war games" since the end of the Soviet Union, which are said to involve nearly 300,000 troops, more than 1,000 aircraft, 36,000 tanks and military vehicles, and dozens of warships.
Most experts say the figure of 300,000 is likely an exaggeration with not all of those actually taking part in the field exercises.
Vostok 2018 comes amid increasing tensions between Moscow and the West over Russia's involvement in the conflicts in Ukraine and Syria. "There is also an element of theatre in this exercise".
The drills, however, allow Russian Federation to practice mobilizing and effectively transporting a large number of forces.
Watling, an expert at RUSI, said the drills also help Russian Federation and China develop their capacity to make small-scale interventions in hotspots overseas, as Russian Federation already appears to be doing in the Central African Republic, as a means to have more heft on the world's stage.
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