South African Nuclear Energy Corporation (NECSA), which tested the material, confirmed that the substance was unenriched uranium, and added it likely came from somewhere where enrichment is taking place.
"Yes it is uranium and the tests suggest that it must have come from a country that is dealing with some uranium enrichment at the moment, very very unlikely (in) Africa," NECSA spokesman Elliot Mulane told AFP.
Mulane said at this stage there was no firm indication of where the uranium could have come from, simply suggesting investigations will now move "across international borders."
Two men in their early 20s were arrested in possession of the 2.2 pounds of uranium while allegedly trying to sell it in the southeastern coastal city of Durban on November 14, officials said.
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The seizure, which also included 90 ecstasy tablets, has heightened fears over the illicit trade in enriched materials that could be used in dirty bombs.
International atomic bodies have been informed of the matter.
According to test results seen by AFP the uranium is 0.38 per cent made up of the U-235 isotope, which is split in a fission reaction.
Mulane said the level of uranium found in the sample is harmless and similar to that found in material used to build hulls of ships and in the construction of airplanes.
Uranium trafficking is rare.