...on Thursday.
Police seized a kilo (2.2 lbs) of the material and arrested three people on Wednesday in an operation along the two central European countries' common border, close to Ukraine.
"It is uranium 235 and 238 (types)," the source said.
The source could not confirm a report carried by Slovak news agency SITA that it was enriched uranium.
Enrichment raises the proportion of the 235-type uranium in the material, which can yield fuel for nuclear power stations or be used to make nuclear warheads. Uranium must be highly enriched to be suitable for use in weapons.
Slovak authorities have called a news conference for 2 p.m. (1300 GMT) on Thursday. They said the material was intended for illegal sale for $1.0 million.
The International Atomic Energy Agency (IAEA), a watchdog for the nuclear industry, says it has recorded 1,250 cases of smuggling and other incidents related to the handling of radioactive material since the 1990s.
Nuclear material is often smuggled from the countries of the former Soviet Union.
Police in Slovakia's former federation partner, the Czech Republic, found 2.73 kg of high-enriched uranium in the capital Prague in 1994, intended for illegal sale.
It was the second biggest ever seizure of high-enriched uranium after a 1994 find of 2.97 kg in the Russian city of St. Petersburg, according to the IAEA. (Reporting by Martin Santa, Writing by Jan Lopatka; Editing by Keith Weir)
Keywords: SLOVAKIA RADIOACTIVE/
[BRATISLAVA/Reuters/Finance.cz]