The agency, Rosatom, said five people killed in the blast were its staff members, and the accident involved "isotope power sources", giving no further details. The tests involved "isotope power sources" on a liquid propulsion system, the agency said, though it added nothing about a radiation release.
There have been "no harmful chemicals released into the atmosphere", the Defense Ministry said, adding that "radiation levels are normal".
The post was later taken down and the defence ministry said radiation levels were normal after the accident.
The nearby city of Severodvinsk recorded elevated radiation levels following the accident and panicked residents rushed to buy iodine to counteract radiation.
It said the run on iodine had occurred in the northern port cities of Arkhangelsk and Severodvinsk and that several pharmacies had run out.
Professor Jeffrey Lewis of the Middlebury Institute of International Studies said his "working hypothesis" was that the blast "was related to Russia's nuclear-powered cruise missile, the 9M730 Burevestnik (NATO name: SSC-X-9 Skyfall)".
In separate interviews, the experts asserted that a liquid rocket-propellant explosion would not release radiation.
USA -based nuclear experts said they suspected the explosion occurred during the testing of a nuclear-powered cruise missile vaunted by President Vladimir Putin past year.
The decree identified the five victims as staff of the Russian Federal Nuclear Centre.
A senior US official told Reuters that the details of the mishap are still unclear, but he expressed skepticism over the official explanation.
It is the second accident involving Russia's military this week.
The blast near Severodvinsk is one of the three incidents which took place in Russian Federation this week as two explosions rocked an ammunition storage facility in the Krasnoyarsk region injuring dozens.
A state commission is investigating the accident. Later regional authorities said that six more people have been hospitalized after the accident.
The military was also shaken by the deaths of 14 sailors killed in a fire on one of the navy's research submersibles in July 1. The defense ministry initially reported that no radiation change was detected, but officials in Severodvinsk later contradicted those claims in a statement that was taken down on Friday without explanation.