Russia's nuclear energy agency said an explosion that caused radiation levels to rise in the Arkhangelsk region was caused by an accident during a test of an "isotopic power source for a liquid-powered rocket engine".
In a statement issued late on Friday, Rosatom said five of his employees had died as a result of the accident and three others were being treated for burns.
The statement was the first confirmation that the agency was involved in the incident, which quickly led to radiation levels up to 20 times higher than those in neighboring Severodvinsk.
Rosatom's description of the incident may indicate that he was testing the Burevestnik nuclear cruise missile mentioned during a speech by Vladimir Putin last year.
Russia's defense ministry first confirmed the blast on Thursday, saying two people were killed and six injured in a failed test of a liquid-powered rocket engine. The injured included ministry officials and civilian contractors.
Rosatom's statement may indicate that three of the first reported as wounded have since died. Despite reports that the injured were transported to Moscow for medical care, neither their names nor their locations were confirmed.
The explosion caused radiation levels to rise for about half an hour in Severodvinsk, contradicting the Ministry of Defense on the grounds that there was no increase in radiation levels. Severodvinsk removed a statement confirming the increase in radiation levels on his site, saying the ministry was managing the response to the incident.
While local authorities called for calm, residents of several cities stocked up on iodine, which is often used to limit the effects of radiation exposure.
Russia also closed a bay in the White Sea for civil traffic for a month, leading to speculation that either the water was contaminated or a search operation was being launched.