The military unit where the fire occurred is located in the village of Nenoksa, near Arkhangelsk, where ballistic missiles are tested for the Russian Navy.
The blast at the military training ground near Arkhangelsk was due to a jet engine exploding, the press service of the Ministry of Defence said.
A source told RIA News in Russia: “Hospitals in the Arkhangelsk region are ready for the possible reception of the victims of the incident near Nenoksa.”
Russian officials initially tried to play down the radiation leak, saying the levels were normal.
A statement from the ministry said: “During testing of a liquid jet engine an explosion and combustion of the product occurred.
“As a result of the event, six defence ministry officials and developer company representatives received injuries of varying severity. Two specialists died from their injuries.
“There have been no harmful chemicals released into the atmosphere, the radiation levels are normal.”
But regional authorities later admitted there had been a “short-term” spike in radiation levels after the explosion.
The Russian city of Arkhangelsk is the north-east of Russia and was once the country’s main seaport until 1703.
Last Monday a massive fire erupted at a military ammunition depot near Achinsk in eastern Siberia’s Krasnoyarsk region.
The blaze triggered a number of explosions that continued for around 16 hours, leaving one person dead and injuring 13 more while more than 16,500 people had to be evacuated from their homes.
Earlier this year 14 people were killed during a fire on a Russian submarinewhich was a nuclear spy vessel thought to be on a secret mission.