By Tom Balmforth and Maria Kiselyova

MOSCOW, Aug 9 (Reuters) - Residents of two northern Russian cities are stocking up on iodine that is used to reduce the effects of radiation exposure after a mysterious accident on a nearby military testing site, regional media reported.

The Ministry of Defence has given few details of the accident, saying only that two people were killed and six injured by the explosion of a liquid-propelled rocket engine at a test site in Russia's north.

Although the ministry initially said no harmful chemicals were released into the atmosphere and radiation levels were unchanged, authorities in the nearby city of Severodvinsk reported what they described as a brief spike in radiation. No official explanation has been given for why such an accident would cause radiation to spike.

"Everyone has been calling asking about iodine all day," one pharmacy was quoted as saying by 29.Ru, a media outlet that covers the Arkhangelsk area.

It said the run on iodine had occurred in the northern port cities of Arkhangelsk and Severodvinsk and that several pharmacies had run out. Severodvinsk is the site of a shipyard that builds nuclear-powered submarines.

"We still have iodine left ... but a really large number of people have come in for it today," another pharmacy was quoted as saying.

Authorities have shut down an area of the Dvina Bay in the White Sea to shipping for a month near the accident site, without explaining why.

An unidentified naval officer quoted by Kommersant newspaper said the accident could have occurred at a testing site at sea and that the explosion of a rocket could have caused a toxic fuel spill.

Russia media have said that the rocket engine explosion may have occurred at a weapons testing area near the village of Nyonoksa in Arkhangelsk region.

Those reports say an area near Nyonoksa is used for tests on weapons, including ballistic and cruise missiles that are used by the Russian navy. Some reports have speculated that the test may have involved a new hypersonic missile called Tsirkon.

Greenpeace cited data from the Emergencies Ministry that it said showed radiation levels had risen 20 times above the normal level in Severodvinsk around 30 kilometres (18 miles) from Nyonoksa. (Reporting by Maria Kiselyova; Writing by Tom Balmforth; Editing by Peter Graff)

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