A gang arrested by Slovakian police was trafficking uranium so enriched that it could have been used by terrorists in a dirty bomb, it emerged today.

Two Hungarians and a Ukrainian man were arrested as they tried to sell the uranium last night. The consignment had been tracked by police after it came to their attention inside the former Soviet Union.

A total of 481.4 grams of uranium was found and investigators believe it contained 98.6 per cent uranium-235. Uranium is considered weapons-grade if it contains at least 85 per cent uranium-235.

"According to preliminary information, the material could have been used to make a so-called dirty bomb," Michal Kopcik, a senior Slovakian police official told a news conference today.

He added that the material contained uranium-235, the type used in nuclear reactors and in nuclear warheads, as well as the naturally occurring uranium-238. While it could have been used to make a ‘dirty bomb’ it could not have made an atomic weapon.

"The radioactive uranium was even more dangerous because it was in powder form," Mr Kopcik explained.

Police said three suspects were detained yesterday near the Hungarian border. The men were carrying two containers filled with the radioactive uranium.

One was a 40-year-old Ukrainian man, another a 49-year-old Hungarian man both living in Ukraine. The third man was aged 51 and from Hungary.

Police said they first traced the radioactive material somewhere in the former Soviet Union but did not give further details about its origin.

"Police were watching the sellers as well as the buyers," Kopcik said, but refused to disclose more information on the buyers. The radioactive material was due to be sold for $3,500 a gramme, he added.

It is the latest in a series of cases involving the sale of radioactive material in Slovakia and the neighbouring Czech Republic.

In 2004, two former Slovak army officers were found guilty by a Czech court of attempting to sell uranium. The two were arrested at a hotel outside Brno in November 2003 after they tried to sell three kilogrammes of the substance.

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