Antinuclear

Rostam added that the explosion took place during the testing of an “isotope power source”.

The official said five of its employees had died as a result of the accident and three more were being treated for burns.

However, the extent of the incident and threat of radiation  has not been disclosed, amid growing global concern.

The Archangelsk naval base has been placed under emergency lockdown for a month, with the nearby White Sea also closed to commercial shipping.

A sudden radiation spike detected in the region following the explosion prompted the initial speculation that the incident was related to a nuclear missile test.

The radiation level was recorded as 20 times higher than the normal level in the nearby city of Severodvinsk.

This has been reinforced by chilling footage filmed in the aftermath of the incident.

One video showed hospital workers wearing hazmat suits while they loaded the injured into an ambulance. Another terrifying video revealed a security escort of ambulances transporting the injured to Moscow.

In this footage, one of the ambulance is clearly coated in a chemical protection film.

A defence ministry source said that the worker’s clothes had been burned as soon as they were hospitalised with suspected radiation. Experts have linked the incident to the testing of the new nuclear-powered cruise missile Burevestnik mentioned during a speech by Vladimir Putin last year.

Local people have reportedly been urged to take precautions against radiation, with children from local kindergartens taken indoors after the blast.

There has also been a rush to buy iodine in Russia’s far north.

Russian expert Dr Mark Galeotti said the incident was “clearly a bigger issue than the Russians are letting on”.

He told the BBC: “Despite what the Kremlin have said, there must have been some sort of radiation leak – and they want people to not just stay out of harm’s way, but also don’t want people coming to the site with Geiger Counters.”