Moscow, Russia (CNN) Five employees of Russia's atomic agency have been killed in a blast at a military test site in northern Russia, the state-run Rosatom said Saturday.

The five were killed "while testing a liquid jet propulsion system," Rosatom, which oversees all Russian nuclear projects, said in a statement on its website.

It added three more people were injured in the incident on Thursday and "have burns of varying severity."

The agency told Tass, a Russian state-run news agency, that the explosion occurred during a missile test on an offshore platform near Severodvinsk. The town in the Arkhangelsk region is home to a naval base and a shipyard. Authorities said the port has been closed off after the incident.

"After the tests were completed, the rocket fuel ignited and detonated," Rosatom said, adding that the explosion threw several employees into the sea. "The search continued until there was no hope left (that) survivors will be found," it said, according to Tass.

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