Five members of Russia's state nuclear agency Rosatom were accidently killed during tests on a military site in northern Russia, RIA news agency quoted Rosatom as saying on Saturday August 10th.
Rosatom said the incident occurred while testing liquid propellant rocket engine, according to RIA.
Three of the Russian agency staff have suffered serious injuries including burns following the incident, Rosatom said.
The Russian Defence Ministry said that the explosion of a liquid-propellant rocket engine has killed two people on Thursday August 8th, AFP reported.
"During the test of a liquid propellant jet engine, an explosion occurred and the equipment caught fire," the ministry said.
"As a result of the accident, six defence ministry employees and a developer were injured. Two specialists died of their wounds," it said.
An initial statement by the defence ministry said the radiation levels near the facility, a ballistic missile test site, in the northern Arkhangelsk region, were "normal," and there was no radioactive contamination.
"No harmful substances were emitted into the atmosphere, the background radiation is normal," the statement said.
However, the city hall announced later that radiation levels briefly rose.
"A short-term spike in background radiation levels was recorded at 12 o'clock in Severodvinsk," the state-run TASS news agency, cited Ksenia Yudina, a spokeswoman for city authorities in Severodvinsk as saying.
Two nuclear experts in the US told Reuters, separately, on Friday August 9th they suspected the accident occurred during the testing of a nuclear-powered cruise missile, which the Russian leader vaunted last year.
"Liquid fuel missile engines exploding do not give off radiation, and we know that the Russians are working on some kind of nuclear propulsion for a cruise missile," said Ankit Panda, an adjunct senior fellow with the Federation of American Scientists.
Speaking on condition of anonymity, a senior US official expressed deep skepticism over Moscow's explanation, saying he would not confirm or deny that a mishap involving a nuclear-powered cruise missile occurred.
"We continue to monitor the events in the Russian far north but Moscow's assurances that 'everything is normal' ring hollow to us," said the official.
"This reminds us of a string of incidents dating back to Chernobyl that call into question whether the Kremlin prioritizes the welfare of the Russian people above maintaining its own grip on power and its control over weak corruption streams."