Russia has seized a consignment of radioactive material found in the luggage of a passenger bound for Tehran at Moscow's Sheremetyevo international airport, the customs service said.
The service said in a statement that tests showed the material was a radioactive isotope Sodium-22 which could be obtained only "as a result of a nuclear reactor's operations".
"A criminal enquiry has been opened and the materials transferred to prosecutors," the statement added.
Customs had been alerted by a warning system at the airport ahead of the Moscow to Tehran flight that background radiation in the departures hall was 20 times the norm. A passenger's bag was then searched.
"Eighteen metallic objects of industrial origin were found, packed into individual steel boxes," it said.
"Tests then found that the objects were in fact the radioactive isotope Sodium-22 that had been machine-produced."
Sodium-22 is a radioactive isotope of sodium that can be used in medical equipment.
No further details were immediately available on the consignment or the identity of the passenger who was carrying the materials.
An official in Iran's embassy in Moscow meanwhile told the Iranian news agency ISNA on Friday that: "The news of the discovery of a radioactive consignment headed for Iran in Moscow is a lie".
Western reports on the seizure were aimed at "sabotaging Russo-Iranian relations and trying to create problems and tensions in relations by fabricating issues," the unnamed official was quoted as saying.
Russia has relatively close ties with Iran and has built its first nuclear power station in the southern city of Bushehr. Moscow has also delivered the nuclear fuel for the reactor.
Moscow has echoed Western concerns about the nature of the Iranian nuclear programme but has stopped short of publicly accusing Tehran of seeking atomic weapons and always said that the standoff should be solved by diplomacy.