Russia has launched its biggest military exercise since the Cold War, involving about 0.3 million (300,000) service personnel, in eastern Siberia.
China is sending 3,200 troops to take part in "Vostok-2018", with many Chinese armoured vehicles and aircraft. Mongolia is also sending some units.
The last Russian exercise of similar scale was in 1981, during the Cold War, but Vostok-2018 involves more troops, according to a BBC report Tuesday.
The week-long manoeuvres come at a time of heightened NATO-Russia tensions.
As the exercises began, Russian President Vladimir Putin met his Chinese counterpart Xi Jinping at a forum in the eastern city of Vladivostok and told him "we have a trusting relationship in the sphere of politics, security and defence".
Relations between Russia and NATO - a 29-member defence alliance dominated by the US - have worsened since Russia annexed Crimea from Ukraine in 2014.
Kremlin spokesman Dmitry Peskov said the drills were justified given "aggressive and unfriendly" attitudes towards Russia.
What will happen in the drills?
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, Gen Valery Gerasimov, was quoted as saying.
The Russian defence ministry says 36,000 tanks, armoured personnel carriers and armoured infantry vehicles will take part in Vostok-2018, from 11 to 17 September, along with more than 1,000 aircraft. Vostok is Russian word 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 practise 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 refuelling 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.
