The gunpowder plant located in the Tambov region southeast of Moscow issued a statement saying that an explosion took place during assembly work involving a contractor.

"Twelve people were injured and four of them died," Tambov Gunpowder Plant said in a statement carried by Russian news agencies. Most of the victims were employees of the contractor.

The plant said it was looking into the reasons behind the explosion, adding that "the human factor" played a role.

Tambov governor Maxim Yegorov ruled out a "terror attack."

The Investigative Committee, which probes major crimes, said that it opened a investigation into violation of safety rules.

Since President Vladimir Putin sent troops to Ukraine in February 2022, Moscow has been on high alert, and there have been a number of drone attacks on installations deep inside Russian territory.

Military factories have also been running at full throttle in Russia since the start of the offensive in Ukraine.