Authorities in the nearby city of Severodvinsk said radiation levels briefly rose after the blast, but the defence ministry said levels were "normal".

"During the test of a liquid propellant jet engine, an explosion occurred and the equipment caught fire," the ministry said of the incident in the small town of Nyonoska.

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

"There's no radioactive contamination," a spokeswoman for the Arkhangelsk region where the base 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. Currently the levels have normalised" at 11:50 am (0850 GMT) .

The accident is the second to hit the Russian military in less than a week, after a fire broke out in an ammunition depot in Siberia on Monday, 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.