A mystery explosion rocked a Russian tank factory which produces engines and artillery for the war in Ukraine.

Online footage showed a fireball engulfing the Chelyabinsk Tractor Plant in Chelyabinsk, a city near the border with Kazakhstan.

The factory is under sanctions from the Ukraine and the US as “an enterprise specialising in the production of diesel engines for military equipment”.

Ukraine has not commented but if responsible, it would be its furthest attack behind enemy lines of the war so far.

But Russian officials have insisted the huge blaze was caused by a short circuit.

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