Several kilograms (pounds) of low-enriched uranium and a few grams (ounces) of weapons-grade material have been stolen from various Russian nuclear sites over the past decade.
The head of the federal nuclear and radioactive security oversight agency said new investments were needed to strengthen safeguards at Russia's nuclear sites and keep the dangerous material out of wrong hands.
"There have been cases of leakage" over the past decade, Yury Vishnyevsky said using a slang term for material being stolen by thieves.
"We are talking about grams (ounces) of weapons-grade materials, and kilograms of low-grade uranium used as fuel by nuclear power plants," the official said Thursday.
He said the losses were most frequently recorded at secretive industrial plants like Electroplate, near Moscow, and the Novosibirsk chemical plant.
Both of these sites are believed to be used for both civilian and military purposes.
Vishnyevsky said Russia need to invest some six billion rubles (190 million dollars) to make these and other sites safe from thieves.
The investment is needed "to modernize technical defense equipment, as well as for preparing and arming the security services at nuclear sites."
Russian officials have in the past denied a series of Western press reports alleging that radioactive material has been stolen from Russian nuclear sites.
Several of these thefts have been pinned by the Western media to people linked to separatists in rebel Chechnya.