Russian authorities have told people in a village near the site of a deadly missile explosion involving a nuclear power test to "leave the territory", after the accident triggered a spike in radiation.

The country's state weather service said radiation levels spiked in the Russian city of Severodvinsk by up to 16 times last Thursday after what officials say was an explosion during a rocket engine test on a sea platform.

"We have received a notification ... about the planned activities of the military authorities. In this regard, residents of Nyonoksa were asked to leave the territory of the village from 14 August," authorities in Severodvinsk were quoted as saying by the Interfax news agency.

It comes following an accident still cloaked in much secrecy, but which the nuclear agency Rosatom said occurred while engineers in Arkhangelsk, were testing a “nuclear isotope power source” for a rocket engine, a tragedy that fuelled radiation fears and raised questions about a secretive weapons programme.

The accident, which occurred on the coast around 600 miles north of Moscow, killed five Russian nuclear engineers, who were laid to rest in funerals attended by thousands of mourners.

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On Tuesday, Russia's TASS news agency revealed medics who treated victims of the accident were sent to Moscow for medical examination.

The medics have signed an agreement promising not to divulge information about the incident, TASS cited an unnamed medical source as saying.

The Defence Ministry initially reported the explosion at the navy's testing range, near the village of Nyonoksa in northwestern Arkhangelsk, killed two people and injured six others.

State-controlled Rosatom then said over the weekend the blast also killed five of its workers and injured three others. It is not clear what the final death toll is.

The company said the victims were on a sea platform testing a rocket engine and were thrown into the sea by an explosion.

Rosatom director Alexei Likhachev praised the victims as "true heroes" and the "pride of our country".

"Our further work on new weapons that we will certainly complete will be the best tribute to them," Mr Likhachev said during the funeral, according to Rosatom. "We will fulfil the Motherland's orders and fully protect its security."

Following the explosion, Russian authorities also closed part of Dvina Bay on the White Sea to shipping for a month, in what could be an attempt to prevent outsiders from seeing an operation to recover the missile debris.

The Severodvinsk city administration said the radiation level rose to 2 microsieverts per hour for about 30 minutes on Thursday before returning to the area's natural level of 0.1 microsieverts per hour. Emergency officials issued a warning to all workers to stay indoors and close the windows.

The radiation level of 2 microsieverts per hour is only slightly higher than the natural background radiation, which could vary between 0.1 and 0.4 microsieverts per hour. It is lower than the cosmic radiation that plane passengers are exposed to on longer haul flights.

Regional authorities have not reported any radiation increases after Thursday's spike.

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Russian environmental groups have urged the government to release details of the radioactive leak, but officials offered no further details.

Neither the Defence Ministry nor Rosatom mentioned the type of rocket that exploded during the test, saying only that it had liquid propellant.

But Rosatom's mention of a "nuclear isotope power source" led some Russian media to conclude it was the Burevestnik (Petrel), a nuclear-powered cruise missile first revealed by Russian president Vladimir Putin in March 2018 during his state of the nation address along with other doomsday weapons.

While presenting the new missile, Mr Putin claimed it will have an unlimited range, allowing it to circle the globe unnoticed, bypassing the enemy's missile defence assets to strike undetected.

The president claimed the missile had successfully undergone the first tests, but observers were sceptical, arguing that such a weapon could be very difficult to handle and harmful to the environment.

Some reports suggested previous tests of the Burevestnik missile had been conducted on the barren Arctic archipelago of Novaya Zemlya and the Kapustin Yar testing range in southern Russia before they were moved to Nyonoksa.

Moving the tests from unpopulated areas to a range close to a big city may reflect the military's increased confidence in the new weapon.