Footage showed a fireball engulfing the Chelyabinsk Tractor Plant, in the industrial city of Chelyabinsk near the border with Kazakhstan, on Sunday night.
Fire engines were photographed in the vicinity of the plant as smoke billowed into the night and firefighters doused the factory with water.
Russian officials said the huge blaze was caused by an oil transformer which exploded.
Ukraine has not commented, but if it is responsible, it would be its furthest attack behind enemy lines of the war so far. Chelyabinsk is more than 1,100 miles from the current front line.
“An oil transformer was on fire, with fire spreading over an area of 40 square metres,” the Russian ministry of emergency situations said.
“Twenty firefighters and six pieces of equipment [fire engines] were involved in the extinguishment.”
There has been no official comment on the number of casualties.
Explosion triggered by short circuit
The Chelyabinsk factory is under sanctions from Ukraine and the United States as “an enterprise specialising in the production of diesel engines for military equipment”.
It produces engines for T72 and T90 tanks, Terminator armoured vehicles and a number of self-propelled artillery systems, the Moscow Times reported.
Witnesses reported hearing an explosion just before the fire broke out, Russian Telegram channel Baza said.
Mash, a Telegram-based Russian newspaper, said power cuts hit nearby buildings in the aftermath and two ambulances attended the scene.
A third Telegram channel, 112, said there were no casualties and the transformer explosion was triggered by a short circuit.
The explosion came the day after Ukraine launched an armada of 35 drones into Russia territory in retaliation for a dawn raid by 70 Iranian-made Shahed drones on Saturday, the largest Russian drone attack of the war so far.
While Ukraine has launched increasingly daring raids behind enemy lines in recent months, they have largely been limited to missile strikes on the five regions occupied by Russian forces – Crimea, Zaporizhzhia, Kherson, Luhansk and Donetsk – and drone attacks into Russia itself.