Firefighters stand outside the metro station where a blast went off in St. Petersburg, Russia, on April 3, 2017. (Xinhua/Irina Motina)

MOSCOW, Aug. 31 (Xinhua) -- Three people were killed in an explosion at the Sverdlov munitions plant in the Nizhny Novgorod region in central Russia, the Russian Emergencies Ministry said Friday.

The explosion occurred during the disposal of obsolete mines supplied by the Russian Defense Ministry in damaged boxes, which had been transferred to the factory in the city of Dzerzhinsk for destruction, according to the unofficial Nizhny Novgorod online city portal.

Three people were killed and four others were hospitalized, of which two were in serious condition, head of the regional division of the Russian Emergencies Ministry Valeri Sinkov said, according to footage published on the division's website.

A fire caused by the explosion has been kept from spreading and the damage has been contained, a statement by the emergencies ministry said.

Investigators are working at the scene, trying to establish the circumstances of the accident, the Nizhny Novgorod regional division of Russia's Investigative Committee said in a statement.