According to a statement by the Russian defense ministry, “Six defense ministry employees and a developer” were injured after a rocket engine caught fire during its test and then exploded.
Two specialists also died of their wounds, it added.
The incident took place on Thursday in the northern Arkhangelsk region of Russia.
A brief rise in radiation was recorded in the nearby city of Severodvinsk after the explosion, reports by Russian media said.
However, a spokeswoman for Arkhangelsk region said there had been “no radioactive contamination.”
The defense ministry also confirmed this and said "no harmful substances" were emitted into the atmosphere and that the background radiation was normal.
The accident is the second to hit the Russian military in less than a week, after a fire broke out in an ammunition depot in Siberia on Monday, causing huge explosions.
At least one person was killed and eight injured while thousands were evacuated from their homes following the blaze at the depot in Krasnoyarsk region's city of Achinsk.