MOSCOW, Russia - A massive explosion rocked one of Russia's biggest munitions factories and left three people dead, and five others with grave injuries on Friday. 

According to Russian media reports, the explosion occurred at the Defense Factory in Central Russia's Nizhny Novgorod region.

Initially, the police said that three people were missing, but later added that nine people were believed to have been working in the area where the explosion occurred. 

While three people were killed, six others were believed to have suffered grave injuries. 

The Ya.M. Sverdlov plant, which is one of the biggest munitions factories in Russia is located in the city of Dzerzinsk.

Police sources quoted in a Sputnik report added that the explosion at the plant triggered a huge fire that soon engulfed a 100-square-meter area.

The report had earlier quoted a source as saying, "The incident occurred at a workshop specializing in the production of ammonal at the plant named after Ya.M. Sverdlov in the city of Dzerzhinsk. Two people were injured.

Another source added in a Tass news report that some ammonal explosive had gone off in one of the plant's workshops.

Further explained,  "A technological process disruption is believed to have caused the explosion. The blaze has been categorized as a three-alarm fire."

Later, officials confirmed that the blast, that was followed by a fire had been successfully extinguished by firefighters. 

The industrial facility, which produces explosives for Russia's Defense Ministry, also caters to the mining industry, and provides oil and natural-gas operations.

It is said to have been involved in the production of a wide range of industrial explosive materials, detonators, cumulative charges for crushing oversized materials in the mining industry as well as perforating charges for the oil and gas industry.