Russia's state nuclear agency Rosatom said early on Saturday that five of its staff members had been killed in an accident during tests on a military site in northern Russia, the RIA news agency reported.
The accident occurred during tests on a liquid propellant rocket engine, the agency said, citing Rosatom.
Russian authorities had previously said that two people had been killed in the incident and that a nearby city had reported a rise in radiation levels when a rocket engine blew up at a testing site in the Archangelsk region on Thursday.
Authorities said they had been forced to shut down part of a bay in the White Sea to shipping.
Local residents have been stocking up iodine used to reduce the effects of radiation exposure after the accident, regional media have reported.Russia's Ministry of Defense has given few details of the accident.