According to the Guardian, the Rosatom statement may indicate that several employees previously named as injured in a ministry of defence statement (which put the toll at two dead, six injured) have died, and the agency did not elaborate on where the workers are being treated.
Officials have raised the death toll to five people in a mysterious August 8 explosion and fire at a military unit in Russia's northwestern Arkhangelsk region, as a string of blasts have rocked Russian military sites in recent days.
Rosatom said its staff were providing engineering and technical support for the "isotope power source" of the engine being tested.
There were no further details of the rocket or fuel type.
It is said that after this accident Radiation has spread in the city of Severodvinsk, 47 kilometers from Ninoshka.
The Russian Defense Ministry said at the time that two people died while testing the liquid jet propulsion system and that no risky substances were released into the atmosphere.
Port authorities on Friday declared the Dvinsky Bay near Severodvinsk closed to shipping until September 10 at the request of the military, the Fontanka.ru news website reported, citing local officials who gave no explanation.
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The explosion caused radiation levels to rise for about half an hour in Severodvinsk, contradicting official ministry of defence claims that there had been no rise in radiation levels. It did not say why.
However, a local news outlet said that residents in nearby areas rushed to buy medical iodine, with pharmacies' stocks reported to be running out in the cities of Arkhangelsk and Severodvinsk, the BBC reported.
A view shows an entrance checkpoint of a military garrison located near the village of Nyonoksa in Arkhangelsk Region, Russia October 7, 2018.
Rosatom said the accident occurred during tests on a liquid propellant rocket engine on Thursday, RIA reported.
There have been "no harmful chemicals released into the atmosphere", the Defense Ministry said, adding that "radiation levels are normal".
Russian maritime authorities also ordered shipping halted in the Dvina Bay region of the White Sea, which is close to the military facility identified as the source of the radiation to shipping, and a specialised vessel used to collect waste from nuclear-fuelled icebreakers was subsequently reported to be in the area, the Times wrote.