Rescue work at the scene of the explosion. (Photo: Russian Emergency Situations Ministry/Reuters)

Hundreds of rescuers are still searching through the rubble at the scene of the blast, located about 320 km southeast of Moscow. Ryazan Governor Pavel Malkov declared a day of mourning, with flags to be lowered on the territory of the whole region.

Earlier, Ryazan region governor Pavel Malkov said that the incident had been triggered by a fire breaking out inside a workshop at the factory.