MOSCOW, Aug. 31 (Xinhua) -- At least three people were killed in an explosion at the Sverdlov defense plant in Russia's Nizhny Novgorod region, Russian media reported Friday.

"According to preliminary reports, the explosion killed three and left another two injured. In addition, another three people were still missing," Tass news agency quoted a regional law enforcement source as saying.

Interfax news agency earlier reported that seven people died in the explosion while Sputnik news agency said four people were hospitalized.

No official death toll has been released so far. Information on the possibility of additional victims is being verified, according to a statement by the Russian Emergencies Ministry's Nizhny Novgorod regional division.

Fire-rescue units are in the process of putting out the fire caused by the explosion, the statement said.