Customs officers said they have seized pieces of radioactive metal from the luggage of an Iranian passenger bound for Tehran from Sheremetyevo Airport.
It was not immediately clear whether the substance could be any use to Iran's controversial nuclear program.
Iran's semi-official news agency, ISNA, confirmed that material had been seized from the luggage of an Iranian passenger in Moscow about a month ago, but denied that it was radioactive.
The Federal Customs Service said in a statement Friday that agents found 18 pieces of metal, packed in steel pencil cases after a radiation alert went off at the airport. It said the gauges showed that radiation levels were 20 times higher than normal.
Spokeswoman Ksenia Grebenkina said the luggage was seized some time ago, but did not specify when. The Iranian wasn't detained, she said, and it was not clear whether he was still in Russia or not. She did not give his name. The pieces contained Sodium-22, she said, a radioactive isotope of sodium that could be produced in a particle accelerator.
Kelly Classic, a health physicist at the United States' renowned Mayo Clinic, said: "You can't make a nuclear bomb or dirty bomb with it."
Iran's ISNA news agency quoted an official at the Iranian Embassy in Moscow as denying that radioactive materials were seized from the luggage of the passenger.
"About a month ago, a misunderstanding arose in connection with a student who was carrying some materials for dentistry uses. The issue was quickly resolved and apologies were offered to him," ISNA quoted the official as saying Friday.
In Washington, State Department spokeswoman Victoria Nuland said officials have contacted Russia for more information and: "Until we hear from the Russians exactly what they've got and how it all went down, I don't think we should evaluate."