A team from the International Atomic Energy Agency is on the ground in Nigeria for what it describes as a "radiological emergency."

Sources say radioactive and highly toxic material that could be used to build a "dirty bomb" was stolen from a Nigerian oil company.

Agency workers arrived Feb. 16, an agency representative said on condition of anonymity.

The Nigerian Nuclear Regulatory Agency admitted earlier this week that devices used to X-ray oil pipelines for cracks were lost, but sources tell Fox News that officials now believe the disappearance of the material was a "strategic theft."

Officials could not say whether the material may have been transported out of the country.

They did not specify how much material was stolen, but they said it was a significant amount of americium and beryllium.

Americium is a synthetic radioactive element produced by the bombardment of plutonium with high-energy neutrons.

Beryllium is not radioactive, but it is a highly toxic metallic element used in aerospace alloys, windows in X-ray tubes, and in nuclear reactors.

Defense and U.S. intelligence officials say many African countries provide safe-haven for terrorist cells, and that Nigeria may be one of them.

Nigerian authorities said earlier this month they were worried the material could fall into the hands of people who might use it for purposes that could threaten public safety and have put all security agencies on alert.

Nigeria is the world's sixth-largest oil exporter, and nearly all of the oil comes from the Niger Delta. But it has no known nuclear program.

Multinational oil companies' facilities in the delta are the object of frequent attacks by saboteurs and thieves. Residents of the impoverished southeastern delta accuse the companies of polluting the land and not returning profits to the area.

The Associated Press contributed to this report.