Five people were killed in an explosion during tests on a military site in northern Russia, the country's state nuclear agency Rosatom said on Saturday.
The accident on Thursday happened during the testing of a liquid propellant rocket engine at a missile test site in the northwestern region of Arkhangelsk, the agency said.
In a statement carried by RIA news agency, Rosatom said three more of its staff had received injuries of varying degrees of seriousness during the accident, including burns.
They were receiving the necessary medical treatment in specialised facilities, it said.
Russian state media had initially reported that a fire occurred on board a vessel at the facility in the port of Severodvinsk in the Arkhangelsk region, an area where nuclear submarines are based.
The Russian defence ministry said two people died while six others were injured when a jet engine exploded.
Authorities said they had been forced to shut down part of a White Sea bay to shipping as a result.
Although the defence ministry said that no harmful chemicals were released into the atmosphere and that radiation levels were unchanged, authorities in the nearby city of Severodvinsk, about 30km from the test site, reported on their website what they described as a brief spike in radiation.
That statement was taken down on Friday without explanation.
Meanwhile, residents have been stocking up iodine - used to reduce the effects of radiation exposure - after the accident, regional media reported.
Two US-based experts said in separate interviews with Reuters news agency that a liquid rocket propellant explosion would not release radiation.
They said they suspected the explosion and the radiation release resulted from a mishap during the testing of a nuclear-powered cruise missile at a facility outside the village of Nyonoksa.
"Liquid fuel missile engines exploding do not give off radiation, and we know that the Russians are working on some kind of nuclear propulsion for a cruise missile," said Ankit Panda, an adjunct senior fellow with the Federation of American Scientists.
Greenpeace has cited data from the Emergencies Ministry that it said showed radiation levels had risen 20 times above the normal level in Severodvinsk, about 30 kilometres from Nyonoksa.
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