Five Rosatom employees were killed and three injured in an explosion at a military testing site in northern Russia on Thursday, RIA news agency cited the state nuclear company as saying.
The accident occurred during tests on a liquid propellant rocket engine, it added.
"Five Rosatom staff members died and a further three people were injured in a tragic accident that took place during tests on a liquid propulsion system involving isotopes at a military facility in Arkhangelsk region.
We offer our deepest condolences, and all possible support, to the families and friends of those who died. Our thoughts are with the loved ones of all those affected," said a Rosatom statement sent to Euronews.
Russian authorities previously said two people had been killed and six injured in the incident at the site in Nyonoksa, near Severodvinsk, in the Arkhangelsk region in the Russian Arctic.
Local authorities in Severodvinsk, which is around 47km east of Nyonoksa, reported a short-term increase in radiation after the blast but they had returned to normal after around 40 minutes.
RIA cited the defence ministry as saying radiation levels were normal after the explosion, which involved a liquid-propellant rocket engine.
No dangerous substances were released into the atmosphere, the government organisation said.
"There is no radioactive contamination," a spokesman for the governor of the Arkhangelsk region told AFP.
However, residents near the site have been buying iodine, which can be used to reduce the effects of radiation exposure, regional media reported.
It was the second of three accidents involving the Russian military this week.
Massive explosions at a Russian military ammunition depot in Siberia on Monday killed one and injured 13 others, prompting the evacuation of thousands.
Russia's Defence Ministry said a fire triggered the explosions at a storage facility for gunpowder charges near the city of Achinsk in eastern Siberia's Krasnoyarsk region.
Authorities declared a state of emergency in the region and ordered the evacuation of residents of several districts in Achinsk, as well as towns and villages within 20km of the depot.
A further two explosions took place at the same site on Friday "during mine clearance and the emergency response," according to the local authority.
It said five employees had been injured, while national media reports said the number of people injured was at least nine.
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