The accident took place at an Arctic military facility on the coast of the White Sea on Thursday, but Russian authorities only admitted its nuclear nature on Saturday.

The explosion caused a spike in radiation levels.

US experts have said it could be linked to testing of the "Burevestnik" cruise missile, touted by President Vladimir Putin earlier this year.

As a memorial service was held for the scientists on Monday, the head of nuclear agency Rosatom said their efforts would continue.

"The best (thing) for their memory will be our further work on the new weapons," Alexei Likhachev was quoted as saying by Russian news agencies.

"We are fulfilling the task of the motherland, its security will be reliably ensured."

Rosatom said this weekend that its staff were providing engineering and technical support for the "isotope power source" of a missile.

The missile was being tested on a platform at sea when its fuel caught fire and caused an explosion, it said. Several staff were blown into the sea by the blast.

The Russian military announced the death of two "specialists" after the explosion but it is not known if they were among the five scientists whose deaths were announced by Rosatom.

Three other people were injured in the accident, suffering burns, according to the nuclear agency.

The military did not initially say that the accident involved nuclear equipment, stressing that radiation levels were normal afterwards.

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

- 'Real heroes' -

In a state of the nation address earlier this year Putin announced the development of what he called "invincible" missiles, threatening to deploy them against "decision-making centres" in Western countries if there were serious threats against Russia.

Experts have linked the blast 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.

The accident took place at the Nyonoksa test site on the White Sea, used for testing missiles deployed in nuclear submarines and ships since the Soviet era.

A memorial service for the scientists was held Monday in the closed city of Sarov, 500 kilometres (300 miles) east of Moscow.

Sergei Kiriyenko, the deputy head of Putin's administration and a former nuclear chief, called the victims "real heroes" at the ceremony.

He said that while conducting the tests the scientists took on physical risks "which, unfortunately, however much you prepare, cannot be completely avoided."

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

It is still a closed city accessible only with special passes.

Last month, 14 Russian navy officers were killed in a fire on a top-secret nuclear-powered submersible in circumstances that have not been fully revealed.

The incident comes with fears of a new arms race after Moscow and Washington withdrew earlier this year from the Cold War-era Intermediate-Range Nuclear Forces (INF) treaty.