Russia has said that five nuclear agency staff were killed by a blast during testing of a nuclear-powered missile at an Arctic military facility on Thursday.

The closed city of Sarov in the Nizhny Novgorod region, about 500 kilometres (300 miles) east of Moscow, announced 24 hours of mourning from 1 pm (1000 GMT) Sunday with flags lowered and entertainment events cancelled, RIA Novosti state news agency reported.

The decree by the city chief said the five killed were staff of the Russian Federal Nuclear Centre.

The Rosatom nuclear agency said memorial services would be held Monday.

During the Cold War, Sarov was a top-secret city known as Arzamas-16. The centre was the birthplace of the Soviet Union's first nuclear weapons.

Sarov is still a closed city only accessible to those with special passes.

Vladimir Putin visited the nuclear research centre in 2012 while campaigning for president.

Rosatom said the accident happened when a missile was being tested on a marine platform in the far northern Arkhangelsk region.

Fuel caught fire and detonated and the force of the blast threw staff into the sea, it said.

Russia's military in an initial statement said two were killed in the accident and it was not known whether those two deaths were among the five that Rosatom reported.

The nearby city of Severodvinsk reported elevated radiation levels following the accident and panicked residents rushed to buy iodine to counteract radiation.

Professor Jeffrey Lewis of the Middlebury Institute of International Studies said his "working hypothesis" was that the blast was related to the 9M730 Burevestnik nuclear-powered cruise missile, known by NATO as SSC-X-9 Skyfall.

Putin said in February that tests on the Burevestnik were going successfully.