He told Reuters both cases had a relatively low level of radioactivity but it was "still enough" to cause radioactive contamination if detonated in a home-made bomb.

"We were alerted by state security who seized the cases, and we were able to confirm that they were radioactive," he said.

Some 50 cases of radioactive uranium and highly radioactive caesium have been seized by Congolese authorities in the central African country over the last four years, said Lumu.

Officials say cases carrying radioactive material are smuggled into the country, from where they are traded across sparsely guarded borders to several of Congo's nine neighbors.

They suspect the cases are brought in to Congo for industrial use in the region's oil and mining sectors, bypassing international conventions on shipping radioactive material.

"It is possible that some of the cases brought in for such use are then stolen or traded into the wrong hands, but we cannot say for sure how many times this has happened," Lumu said.

The two cases seized earlier this month were of a similar size to many others smuggled in and could also have been destined for industrial use.

State authority and border controls are often weak or non-existent in the former Zaire, recovering from over five years of a war that involved several of its neighbors.

Most of the illegal cases seized in the past have been discovered in Kinshasa, although identical cases have also been found in neighboring Uganda and Tanzania, said Lumu.

Oil rich Congo Republic is thought to be the most common destination for the nuclear material, with only the Congo river separating its capital, Brazzaville, from Kinshasa in the DRC.

Last week, police in neighboring Zambia arrested two men in possession of a suspected bomb-grade uranium cache, adding they suspected it came from the Democratic Republic of Congo.