Vladimir Putin's forces have rehearsed using tactical nuclear weapons at an early stage of a major conflict, leaked Kremlin documents have revealed.

The files even show plans for training scenarios if China, an ally of Moscow, launches an invasion.

The threshold for using tactical nuclear weapons is much lower than Russia has publicly admitted, experts have warned.

Around 29 secret Russian files were written up between 2008 and 2014.

Vladimir Putin's forces have rehearsed using tactical nuclear weapons at an early stage of a major conflict, leaked Kremlin documents have revealed

Scenarios included were war-gaming and presentations for naval officers.

The plans referred to discussing operating principles for the use of nuclear weapons.

An enemy incursion on Russian territory or the destruction of 20 per cent of the Kremlin's strategic ballistic missile submarines could trigger a nuclear response.

Alexander Gabuev, director of the Carnegie Russia Eurasia Center in Berlin, told The Financial Times: "The operational threshold for using nuclear weapons is pretty low if the desired result can’t be achieved through conventional means."

Despite dating back more than a decade, experts have also warned the documents remain relevant to current Russian military doctrine.

However, the files also unearth the Kremlin's concerns about China.

The two nations, often shunned by the West, forged a nuclear no-first-strike agreement alliance in 2001.

Sino-Russian relations have only been strengthened since Xi Jinping came into power in 2012.