Earlier, the authorities in a nearby city said the accident had caused a spike in radiation levels but the military denied this.

The accident on Thursday happened during testing of a liquid propellant rocket engine at a missile test site in the far northern Arkhangelsk region.

In a statement, Rosatom said the accident killed five of its staff and injured three, who suffered burns and other injuries.

Rosatom said its staff were providing engineering and technical support for the "isotope power source" of the missile engine.

The military had not described the accident as involving nuclear fuel.

The authorities have released few details of the accident at the Nyonoksa test site on the White Sea, used for testing missiles used in nuclear submarines and ships since the Soviet era.

The defence ministry initially said that six defence ministry employees and a developer were injured while two specialists died of their wounds.

The authorities in Severodvinsk, a city around 30 kilometres (19 miles) away from the test site, said on their website that automatic radiation detection sensors in the city "recorded a brief rise in radiation levels" on Thursday morning.

The post was later taken down.

An official responsible for civil defence, Valentin Magomedov, told TASS state news agency on Thursday that radiation levels rose to 2.0 microsieverts per hour for half an hour from 11:50 am (0850 GMT), before falling sharply.

He said this exceeded the permitted limit of 0.6 microsieverts, TASS reported.

Russian online media published unattributed video that journalists said showed a line of ambulances speeding through Moscow to take the injured to a centre that specialises in the treatment of radiation victims.

Rosatom said that the injured were being treated at a "specialised medical centre."

- Iodine panic -

An expert from Moscow's Institute for Nuclear Research, Boris Zhuikov, told RBK independent news site that isotope power sources are mainly used in spacecraft and are not usually dangerous for people working with them.

"If they are damaged, people who are nearby could be hurt. Isotope sources use various types of fuel: plutonium, promethium or cerium," Zhuikov said.

The radioactivity levels involved are "absolutely not comparable with those during serious accidents at reactors," he added.

The news of the accident prompted residents of Severodvinsk to rush to pharmacies on Thursday to stock up on iodine, which can be taken to stop the thyroid gland absorbing radiation.

"People started to panic. Within a matter of an hour all the iodine and iodine-containing drugs were sold out," pharmacist Yelena Varinskaya told AFP.

The Soviet Union saw the world's worst nuclear accident at Chernobyl in 1986, when the authorities sought to cover up the seriousness of the disaster.