Moldovan police working with the FBI are reported to have stopped four attempts by smugglers to sell nuclear material to extremists in the Middle East over the past five years.
The most recent case was in February when undercover agents were offered a large amount of radioactive caesium, the Associated Press reports.
Investigators say much of the material is believed to come from Russia.
They say some gangs have alleged links to Russia's intelligence services.
Police and judicial authorities in Moldova shared information with AP to highlight how dangerous the nuclear black market has become, the news agency says.
They say the deterioration in relations between Russia and the West has made it more difficult to know whether smugglers are succeeding in selling radioactive material originating from Russia abroad.
Moldova is a former Soviet republic.
"We can expect more of these cases," said Moldovan police officer Constantin Malic, who investigated all four cases.
"As long as the smugglers think they can make big money without getting caught, they will keep doing it."
Moldova - timeline of nuclear smuggling
- 2010: 1.8kg of Uranium-238 seized in Chisinau when three people tried to sell it for €9m (£6.6m; $10m)
- 2011: Six detained for trying sell 1kg of weapons-grade Uranium-235 for €32m; they said they also had access to plutonium
- 2014: Smugglers allegedly tried to sell 200g of Uranium-235 from Russia to undercover security agents for $1.6m; 1.5kg of Uranium-235 seized close to Moldovan border in Ukraine
- 2015: Undercover agent bought ampoule of Caesium-135; materials contaminated with Caesium-137 found in central Chisinau
In many cases seen by AP, deals were broken up by police in the early stages but ringleaders managed to escape - possibly with their nuclear contraband.
In the case involving the Caesium, the would-be smuggler wanted €2.5m (£1.8m) for enough radioactive material to contaminate several city streets.
At a club in the Moldovan capital Chisinau he told a potential client - who was really an informant: "You can make a dirty bomb, which would be perfect for the Islamic State. If you have a connection with them, the business will go smoothly."
A sample vial of less-radioactive Caesium-135 was produced and police pounced, arresting the man and two others.
It is not clear whether the cases in Moldova indicate a more widespread nuclear smuggling operation, the report says.
Eric Lund, spokesman for the US State Department's bureau in charge of non-proliferation said Moldova had taken "many important steps" to strengthen its counter nuclear smuggling capabilities.