Preparations are under way for Russia's biggest military exercise since the Cold War, involving about 300,000 service personnel, in eastern Siberia.

China is sending 3,200 troops to take part in "Vostok-2018", with many Chinese armored vehicles and aircraft. Mongolia is also sending some units, the BBC wrote.

The last Russian exercise of similar scale was in 1981, during the Cold War but Vostok-2018 involves more troops.

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It comes at a time of heightened NATO-Russia tensions.

Relations between Russia and the North Atlantic Treaty Organization (NATO) - a 29-member defense alliance dominated by the U.S. - have worsened since Russia annexed Crimea from Ukraine in 2014.

Read alsoRussia, West show war games aren’t just games: ex-NATO chief

Kremlin spokesman Dmitry Peskov said the drills were justified given "aggressive and unfriendly" attitudes towards Russia.

Tuesday and Wednesday will see planning and preparation while actual operations will start on Thursday and last five days, the head of the Russian general staff, Army General Valery Gerasimov, was quoted as saying by Interfax news agency.

The Russian defense ministry says 36,000 tanks, armored personnel carriers and armored infantry vehicles will take part in Vostok-2018, from 11 to 17 September, along with more than 1,000 aircraft. Vostok is Russian for east.

The exercise will be spread across five army training grounds, four airbases and areas in the Sea of Japan, Bering Straits and Sea of Okhotsk. Up to 80 naval vessels will take part, from two Russian fleets.

The drills will not be near the disputed Kuril islands north of Japan, Russia says.

The ministry's TV channel Zvezda says three brigades of Russian paratroops will play a key role, during drills at the Tsugol military range, near Russia's borders with China and Mongolia.

A key aim is to practice the rapid deployment of thousands of troops, as well as aircraft and vehicles, from western Russia to eastern regions, across thousands of miles, TV Zvezda reports. That involves in-flight refueling of fighter jets.

The scale of Vostok-2018 is equivalent to the forces deployed in one of the big World War Two battles.

A smaller-scale Russia-Belarus exercise was held last year.