Speaking in Helsinki with Finnish President Sauli Niinisto, Putin said Moscow could not reveal everything about the explosion because of its military nature.

He did not specify which weapon system was being tested at the time of the explosion.

The Russian leader stressed the need for a more effective system of information exchange on such accidents in the future.

Russia’s state nuclear Agency said this month that five of its employees were killed and three others wounded in an explosion involving “isotopic power sources” that occurred during a missile test on an offshore platform on August 8.

This week, Putin said there was no risk of rising radiation levels after the explosion. (Reporting by Olesya Astakhova and Anna Kauranen Writing by Gabriel Tetrault-Farber Editing by Andrew Osborn)