A delegation from the global nuclear agency is on its way to Nigeria to help track down radioactive material lost by an oil company.
The radioactive material was used in the oil industry
The two devices contained caesium-137 which could potentially be used in a radioactive "dirty bomb", a source in the International Atomic Energy Agency (IAEA) told a BBC correspondent.
The hand-held devices - used to X-ray oil pipelines to check for cracks - are also a risk to people's health, the source said.
Both devices have been missing since December and seem to have been stolen or fallen off a transport vehicle in the southern Niger Delta region, an official at the Nigerian Nuclear Regulatory Authority (NNRA) said.
The Nigerian authorities reported the loss to the IAEA and are now awaiting advice.
Residents of the oil-producing region have been warned not to touch the material, which may be in "thick steel cylindrical containers with yellowish-black" markings.
Health workers have also been urged to keep a look-out for anyone with prolonged nausea or skin burns.
Sabotage and theft
The material went missing on 3 December 2002 but a public warning was only made last week.
"We have... informed the International Atomic Energy Agency in case somebody stole it and wants to take it outside Nigeria," Shams Elegba, head of the NNRA, told the Associated Press news agency.
The material was lost while in transit between Warri and Port Harcourt.
Delta residents say they do not profit from the region's oil
Oil installations in the region are frequently the object of sabotage and theft.
Residents accuse multinational companies and the government of not ploughing oil wealth back into the region.
They also say the oil industry has led to massive pollution in the region.