There are growing problems in Africa with trafficking in materials that could be used to make a dirty bomb, the International Atomic Energy Agency (IAEA) has warned.
A Casablanca hotel was among terrorists' targets
The bombs could be detonated in order to scatter radioactive material.
Last year the agency sent missions to Nigeria and Tanzania to help the authorities there cope with materials seized from traffickers.
The IAEA has warned that if a dirty bomb exploded in a city it would cause complete panic.
"Illicit trafficking in nuclear materials is an increasing problem for states in Africa", a memo presented to the IAEA board in Vienna said, London's Financial Times newspaper reported.
A dirty bomb is a radiological weapon but unlike a nuclear bomb, its purpose is to contaminate rather than destroy.
It uses normal explosives to disperse radioactive materials in the local environment, creating a hazard to health that could last for years unless cleaned up.
The relative ease of making such a bomb means it is a potent terrorist weapon.
The warning by the IAEA comes amid rising international concern about the risks of terrorists seeking to acquire a dirty bomb.
Terrorists killed 15 in attack on Israeli-owned hotel in Mombasa
Terrorists have used Africa in recent years to carry out attacks against western targets.
Five bombs went off within minutes of each other in the centre of in the Moroccan city of Casablanca on 16 May, 2003.
Twelve suicide bombers were among 43 people killed.
In East Africa, Kenya in particular has suffered a number of terrorist attacks widely believed to have been carried out by groups linked to Osama Bin Laden's al-Qaeda network.
These include the bombing of the American embassy in Nairobi in 1998, the 2002 attack on an Israeli-owned hotel in Mombasa and - on the same day - the attempted shooting down of an Israeli airliner near Mombasa Airport.
Eastern Africa countries, including Somalia, Sudan, Djibouti, Kenya, Eritrea, Ethiopia, Tanzania and Uganda, have been classified by the US as terror havens.