The deadly strain of bird flu has been found in poultry in northern Nigeria, the World Organisation for Animal Health (OIE) has said in a statement.
The OIE said the strain detected was "highly pathogenic"
The Paris-based organisation said this was the first time the disease had been detected in Africa.
The body said it was the "highly pathogenic" strain of the H5N1 bird flu virus, which can kill humans.
It was detected on a farm in the northern state of Kaduna, where a team of experts have been sent.
Authorities there said they had taken measures to stamp out the outbreak by disinfecting the affected premises, imposing a quarantine and putting restrictions on animal movements.
It is not clear if the case on a commercial chicken farm in Jaji, near the city of Kaduna, has any relation to the deaths of thousands of chickens in neighbouring Kano state.
Officials at the Ministry of Agriculture say they are still investigating whether the poultry there died of a more common avian disease.
The BBC's Alex Last in Lagos says an outbreak of bird flu could have devastating consequences in Nigeria where millions of people rear chickens as a basic source of income.
The OIE said that an Italian laboratory for avian flu had detected the strain from samples from the infected farm which had some 46,000 birds.
"We are really not dealing with a backyard operation," OIE expert Alex Thiermann told Associated Press news agency.
Farmers are preparing to hold an emergency meeting in Kano, where the price of chickens has halved, with a bird now fetching not more than $2.
Nigerian Agriculture Minister Adamu Bello said the government would cull all infected birds and has announced a multi-million dollar compensation programme for farmers.
But a northern Nigerian farmer told the BBC News website that people fear they will not be compensated.
"The dead birds are being sent to market to be sold as meat... because people are not sure if the government will assist them," said Auwalu Haruna from Kano.
Mr Bello said the bird flu might have been carried by migrating birds or the smuggling of infected chickens from abroad.
For two years Nigeria has banned poultry imports from countries which have experienced cases of bird flu.
There are fears that the disease could easily spread in Africa because of a lack of safeguards.
"What is most important now is not how it got into Nigeria, but how it can be prevented from leaving Nigeria," Cape Town ornithologist Phil Hockey told Reuters.
Mr Haruna said affected farmers from Kano were still waiting to be quarantined.
More than 80 people have died of H5N1 bird flu since the disease's resurgence in December 2003 - most in Asia.
Experts point out that cross-infection to humans is still relatively rare, and usually occurs where people have been in close contact with infected birds.
But they say if the H5N1 strain mutates so it can be passed between humans some 150m people could die.