An explosion occurred during testing of a liquid propellant jet engine at the base near the Arctic, causing equipment to catch fire, the defence ministry said.

"As a result of the accident, six defence ministry employees and a developer were injured. Two specialists died of their wounds," it added.

The incident took place in the small town of Nyonoksa around 1,000 kilometres (615 miles) north of Moscow, which is Russia's main testing centre for ballistic missiles used on naval submarines and warships.

Authorities in the city of Severodvinsk around 30 kilometres (20 miles) away said radiation levels briefly rose after the blast.

The defence ministry said in a brief statement on the accident released to Russian news agencies that radiation levels were "normal" and there were no harmful emissions.

Health officials said that no members of the public were affected.

"There's no radioactive contamination," a spokeswoman for the Arkhangelsk region where the testing centre is located told AFP.

Severodvinsk authorities said on their website that automatic radiation detection sensors in the city "recorded a brief rise in radiation levels" at 11:50 am (0850 GMT). They said levels were below permissible limits by 2 pm.

It was not clear exactly when the explosion occurred but the defence ministry made an official statement on the accident around 1 pm.

State institutions in Severodvinsk received an automatic warning message saying workers should stay in buildings and close windows due to the elevated radiation levels, Russian news agency Interfax reported.

Severodvinsk's Zvyozdochka shipyard builds and repairs nuclear submarines. One caught fire while being repaired there in 2015.

A source told the Vedomosti newspaper that the accident occurred during testing of the engine of a sea-launched missile currently used by the Russian navy.

The testing centre dates back to 1954 and was used by the Soviet Union to test its nuclear arsenal.

The accident comes days after a fire broke out in an ammunition depot in Siberia, causing huge explosions.

At least one person was killed and eight injured while thousands were evacuated from their homes following the blaze at the depot in the Krasnoyarsk region on Monday.

In a previous accident in Nyonoksa in 2015 a cruise missile being tested there fell on a block of flats in the town, starting a fire, but no one was hurt.