An explosion at a Northern Russian base that killed at least five people last week was linked to a small nuclear reactor, state nuclear officials said.

The explosion occurred Thursday on a platform in the White sea off the coast of Nyonoksa and triggered a report of a nearby radiation spike in Severodvinsk.

State nuclear power company Rosatom said the explosion occurred during a test of a “nuclear isotope source” for the missile.

Vyacheslav Solovyov, scientific Director of the Russian Federal nuclear center, also confirmed to a local newspaper that the Institute is studying “small” energy sources, including “fissile materials,” the New York Times and Reuters reported.

Five scientists from the center were buried Monday. The country’s defense Ministry initially said two people were killed in the accident, but Rosatom said five of its workers died.

Nuclear experts believe the accident occurred during a test of a missile that Russian President Vladimir Putin touted last year, the Times and Reuters reported.

The explosion occurred when the rocket’s fuel caught fire, Reuters reported. “Testers are national heroes ,” says Valentin Kostyukov, head of the nuclear center at Rosatom.

Russian military officials denied that the explosion caused a spike in radiation, but the city of Severodvinsk initially posted a statement on its website warning of the rise, which 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 in the Chernobyl disaster in 1986, the Times reported. 

Contribution: Associated Press. Follow Ryan Miller USA today on Twitter @RyanW_Miller