An unidentified naval officer quoted by Kommersant newspaper said the accident could have occurred at a testing site at sea and that the explosion of a rocket could have caused a toxic fuel spill. "The search continued until there was no hope left (that) survivors will be found", it said, according to Tass.
"The tragedy occurred during the period of work related to the engineering and technical support of isotopic power sources in the liquid-propellant system", it said.
The city that hosts Russia's main nuclear research site announced a day of mourning Sunday for five of its staff killed during a missile test that provoked elevated radiation levels. It gave no further explanation, and a spokeswoman at the agency contacted by Reuters declined to clarify.
The statement was the first confirmation that the agency was involved in the incident, which briefly drove radiation levels up to 20 times their normal levels in the nearby city of Severodvinsk.
The post was later taken down and the defence ministry said radiation levels were normal after the accident. But the city of Severodvinsk, which has a population of 185,000, reported a spike in radiation at noon.
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. It did not say why.
Local authorities reported that a short-term increase in radiation was recorded in Severodvinsk.
A source told the Vedomosti newspaper that the accident occurred during testing of an engine used by a sea-launched missile now in use by the Russian navy.
Nyonoksa hosts a navy facility that serves as a base for testing intercontinental ballistic missiles intended for nuclear submarines.