Rosatom said the force of explosion on Thursday threw several of its staff from a testing platform into the sea.

The military had not initially said that the accident involved nuclear equipment, but said that radiation levels were normal afterwards.

Nevertheless, officials in the nearby city of Severodvinsk reported that radiation levels briefly increased after the accident.

The incident occurred in the far northern Arkhangelsk region during testing of a liquid propellant jet engine when an explosion sparked a fire, killing two, the defence ministry said in a brief statement.

It was not immediately clear whether those two deaths were included in the five that Rosatom had reported.

Russian state news agencies quoted a defence ministry source as saying both defence ministry and Rosatom employees were killed.

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

The missile was being tested on a platform at sea when its fuel caught fire and triggered an explosion, Rosatom said in a statement quoted on Russian television.

Several staff were thrown into the sea by the force of the blast, the nuclear agency said, adding that it only announced the deaths when there was no longer any hope that the employees had survived.

The accident left three more of its staff with burns and other injuries, Rosatom said.

It said staff knew of the "potential risk" of the test.

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

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

- Radiation spike -

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

The post was later taken down and the defence ministry said that radiation levels were normal after the accident.

A Severodvinsk official responsible for civil defence, Valentin Magomedov, told TASS state news agency 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.

Greenpeace Russia published a letter from officials at a Moscow nuclear research centre giving the same figure, but saying higher radiation levels lasted for an hour. The officials said this did not present any significant risk to public health.

Russian online media published an unattributed video which 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 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.

News of the accident prompted residents of Severodvinsk to rush to pharmacies to stock up on iodine, which can be taken to prevent 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.