An explosion at a northern Russian base that killed at least five people last week involved a small nuclear reactor, state nuclear officials said.

The blast occurred Thursday on a platform in the White Sea off Nyonoksa and caused a reported nearby radiation spike in Severodvinsk.

State-controlled nuclear energy company Rosatom said the explosion occurred during a test of "a nuclear isotope power source" for a rocket.

Vyacheslav Solovyov, scientific director at the Russian Federal Nuclear Center, also confirmed to a local newspaper that the institute was studying "small-scale" power sources that include "fissile materials," the New York Times and Reuters reported.

Five scientists from the center were buried Monday. The country's defense ministry originally said two people were killed in the accident, but Rosatom said five of its workers were killed.

Nuclear experts believe the accident occurred during a test of a missile that Russian President Vladamir Putin has touted in the past year, the Times and Reuters reported.

The SSC-X-9 Skyfall is a protoype cruise missile that could reach any corner of the world with a nuclear reactor as its power source, the Times reported.

The explosion occurred when the rocket's fuel caught fire, according to Reuters. “The testers are national heroes,” said Valentin Kostyukov, head of a nuclear center part of Rosatom.

Russian military officials denied that the blast caused a radiation spike, but the city of Severodvinsk initially posted a statement on its website warning of the increase that was later taken down, Bloomberg reported.

The extent of the damage from the accident was not immediately clear but appears to be much smaller than that of the Chernobyl disaster in 1986, the Times reported.

Contributing: The Associated Press. Follow USA TODAY's Ryan Miller on Twitter @RyanW_Miller