MOSCOW, Russia - Concerns are mounting over an explosion at a military test site in northern Russia which killed five elite nuclear scientists, and injured 3 others.

The blast took place last Thursday in the city of Sarov, in the Arkhangelsk region.

The scientists killed were employed by Rosatom, Russia's state atomic energy corporation.

In a statement, the corporation said the explosion was a tragic accident, and occurred during tests on a liquid propulsion system involving isotopes.

"Missile tests were held on the sea-based platform. Missile fuel inflammation with further detonation occurred after completion of tests. Several staff members were thrown away to the sea and there was a hope to find them alive. The search continued until the hope to find survivors was present. The death of five Rosatom employees dealing with work related to a radio isotope power source being part of the missile was announced only after that," a further statement from Rosatom said.

"The concourse of factors was present, which often takes place when testing new technologies," a spokesman of Rosatom added.

They named those killed as Alexey Vyushin, Yevgeny Koratayev, Vyacheslav Lipshev, Sergey Pichugin and Vladislav Yanovsky.

Sarov, where the blast occurred is a closed city. It is off-limits to foreigners, and can only be entered with a special permit

There are fears the test has some nuclear bearing, however officials have not eleaborated on the reasons for the blast, or the purposes of the testing.

What is known is that radiation levels did briefly rise after the blast, according to the Tass news agency.

The Arkhangelsk regional governor confirmed to Tass there would be no evacuation in the aftermath of the blast.

"We are skeptical of the claim that what was being tested was a liquid propellant jet engine," Jeffrey Lewis, an arms-control expert at the Middlebury Institute of International Studies at Monterey, told CNN. "We think it was a nuclear-powered cruise missile that they call Burevestnik."

The same missile is known by NATO members as SCC-X-Skyfall.

A U.S. official has also told CNN that the explosion was "likely" linked to the Skyfall prototype.

U.S. President Donald Trump says the U.S. is learning a lot from the incident.

"The United States is learning much from the failed missile explosion in Russia.We have similar, though more advanced, technology," he posted on Twitter.

"The Russian 'Skyfall' explosion has people worried about the air around the facility, and far beyond. Not good!" he added.

Pictured: City of Sarov | Credit: Wikipedia).