Russian authorities say they are hunting the mastermind of a uranium and plutonium smuggling syndicate that tried to sell the fissile material to a North African country for possible use in a "dirty bomb."

The suspect has been confirmed to be a Russian after Moldovan authorities foiled the sale of 4.4 grams of uranium-235 by a group in the breakaway Republic of Transnistria in June. Six members of the syndicate were, arrested but the ringleader and the North African buyer remain at large.

The uranium seized from the group was not enough to make a nuclear bomb, but it indicated that nuclear materials stolen from Soviet reactors during the breakup of the former Soviet Union are available on the black market.

"A nuclear attack would be much more devastating but much more unlikely. The dirty bomb threat still has a small probability of happening but would be much less catastrophic in terms of casualties and the radiological effect would be minimal," Steven Pifer, a nuclear proliferation expert at the Brookings Institute in Washington, told Deutsche Welle.

Moldovan agents posed as buyers upon learning that the syndicate was selling 9 kilos of enriched uranium for $31 million. Interrogators also learned that the group possessed plutonium.

The group admitted possessing only 1 kilogram of uranium stored in another country, said Maria Vieru of the Moldovan prosecutors' office.

In response to the incident, the U.S. Senate Foreign Relations Committee issued a report that includes a call to Russian authorities to increased vigilance against theft of nuclear materials from its nuclear energy and military facilities.

Russian defense official Vladimir Averchev gave assurances that such thefts would be impossible today.