A kilogramme of uranium seized in South Africa as it was allegedly being sold likely originated from a nuclear enrichment plant outside the continent, the country's nuclear energy agency said Monday.

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."

China, Iran, Japan, North Korea and the United States are among more than a dozen countries involved in uranium enrichment.

Two men in their early 20s were arrested in possession of the kilogramme (2.2 pounds) of uranium while allegedly trying to sell it in the southeastern coastal city of Durban on November 14, officials said.

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 percent made up of the U-235 isotope, which is split in a fission reaction.

That level is well below the average 0.7 percent U-235 found in natural uranium and the 90 percent level needed to be considered weapons grade.

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.

South Africa has the continent's only nuclear power plant, but several countries have significant uranium deposits and mining activities.

The arrested suspects appeared in court last week and were remanded in custody until December 3 for a bail application.