War-ravaged Iraq is also struggling to contain the disease. Police cars and ambulances drove through the streets of the southern city of Amara on Wednesday urging residents to kill their birds to prevent its spread.

So far, victims have contracted the disease through close contact with infected birds. Experts stress cases of human infection are relatively few compared with the millions of birds that have contracted the disease.

However, scientists fear that H5N1 could mutate into a form that passes easily from person to person, sparking a human influenza pandemic.

"The outbreak in Kaduna state in northern Nigeria proves that no country is risk-free and that we are facing a serious international crisis," said Samuel Jutzi, a director of the UN's Food and Agriculture Organisation (FAO) in Rome.

"If the situation in Nigeria gets out of control, it will have a devastating impact on the poultry population in the region, it will seriously damage the livelihoods of millions of people and it will increase the exposure of humans to the virus," Jutzi added.

Nigerian Agriculture Minister Adamu Bello said there had not been any poultry workers infected so far. However, it would be difficult for authorities to know this for sure as mortality rates in impoverished Nigeria are among the highest in the world and people are often buried without any formal medical check.

World Health Organisation (WHO) regional adviser Adamou Yada noted there had not yet been any known cases among humans in Africa.

"In Africa we have so many diseases, so many priorities, if it spreads in Africa it will be a nightmare," Yada told Reuters from WHO Africa headquarters in the Congo Republic.

CHINA CHALLENGE

Bird flu is spreading westwards from Asia where it is endemic in poultry across parts of the continent.

China has more poultry than anywhere else in the world and remains a key battleground in fighting the disease.

The latest bird flu patient brought the number of the country's confirmed cases in humans to 11, the Xinhua news agency said. Seven people have died from the virus in China.

The patient, a 26-year-old woman farmer from eastern China's Fujian province, was in a stable condition, Xinhua said, citing a report from the Ministry of Health.

Iraq last month became the latest country to suffer a human death from the virus when a teenage girl in the north died.

Local officials now suspect that the virus has killed a person in the south, dashing hopes it is confined to a small part of the country.

The governor of Maysan province told Reuters the suspected bird flu victim was a 24-year-old pigeon seller from Amara who died on Sunday. The WHO said earlier that Iraqi officials had identified the victim as a 13-year-old boy.

Migratory birds have been blamed for the spread of the virus, but it is not clear how it reached Nigeria, with a poultry population of 140 million.

Experts called for urgent action to halt further spread.

"What is most important now is not how it got into Nigeria but how it can be prevented from leaving Nigeria," said Phil Hockey of the Percy FitzPatrick Institute of African Ornithology in Cape Town.