According to police, the two Hungarians and a Ukrainian had just under half a kilogram (about a pound) of uranium in powder form that investigators believe came from an unspecified ex-Soviet republic.

"It was possible to use it in various ways for terrorist attacks," a senior police official, Michal Kopcik, said.

He said police had intelligence suggesting that the suspects - whose names were not released, but were aged 40, 49 and 51 - originally had planned to sell the material early this week. One of the Hungarians had been living in Ukraine. Police moved in when the sale did not occur as expected, he said.

Kopcik said investigators were still working to determine who ultimately was trying to buy the uranium, which the trio allegedly was selling for $1m (£485,000).

Three other suspects - including a Slovak national identified only as Eugen K - were detained in the neighbouring Czech Republic in mid-October for allegedly trying to sell fake radioactive materials. It was unclear to what degree, if any, they played a role in the thwarted uranium sale.

Police said a total of 481.4g of uranium had been stored in unspecified containers. Investigators concluded that the material consisted of 98.6% uranium-235. Uranium is considered weapons-grade if it contains at least 85% uranium-235.

"According to initial findings, the material originated in the former Soviet republics," Kopcik said.

Western officials have long harboured concerns over the risk of nuclear smuggling from the former Soviet Union, although US-funded safeguarding programmes have reduced the danger of nuclear trading.

Slovakia's border with Ukraine is the EU's easternmost frontier, and authorities have spent millions tightening security in the past few years, amid fears of nuclear smuggling into the EU . In 2003, police in the Czech Republic, which borders Slovakia, arrested two Slovaks in a sting operation in the city of Brno after they allegedly sold undercover officers natural depleted uranium for $715,000.

Slovak and Hungarian police worked together on the new case for several months, said Martin Korch, a Slovak police spokesman. He would not say how long the suspects were under surveillance, or give details about the arrests and to whom they were trying to sell the material. The Vienna-based International Atomic Energy Agency, the UN nuclear watchdog, said that last year alone there were 252 reported cases of radioactive materials that were stolen, missing, smuggled or in the possession of unauthorised individuals - a 385% increase since 2002.

But the IAEA cautioned that the jump was due at least in part to better reporting and improved law enforcement efforts. Of the 252 cases, about 85 involved thefts or losses, and not all the material was suitable for use in a weapon.

The US-based Nuclear Threat Initiative, an organisation dedicated to reducing the global threat from nuclear weapons, reported last year that Russia remains the principal country of concern for contraband nuclear material, given the decline in security at nuclear-related industries after the collapse of the Soviet Union in 1991.

In 2006, Georgian agents working with CIA officials set up a sting that led to the arrest of a Russian citizen who tried to sell a small amount of weapons-grade uranium that he had in a plastic bag in his jacket pocket.

In 1997, seven men who officials said planned to smuggle 5kg of enriched uranium to Pakistan or China were arrested in the Siberian city of Novosibirsk. That uranium reportedly had been stolen from a plant in the former Soviet republic of Kazakhstan.

Roughly 25kg of highly enriched uranium or plutonium is needed in most instances to make a crude nuclear device. But a tiny fraction of that is enough for a dirty bomb - a weapon designed to sow fear and chaos, rather than inflict human casualties.