In Iraq, a World Health Organization team arrived in Iraqi Kurdistan's biggest city of Suleimaniya to probe the spread of bird flu as new suspected cases emerged in the country.
Nigerian agriculture ministry spokesman Tope Ajakaiye yesterday said tests on chicken carcasses had identified the H5N1 strain, which can kill humans, in northern Nigerian sites more than 200 kilometres (125 miles) apart.
"Four farms have been cordoned off and quarantined," he said.
Yesterday's announcement will increase fears that bird flu may be poised to spread rapidly around the country.
At Sambawa farm in Kaduna state, where the virus was first identified, 20 police officers blocked access to a site where 45,000 chickens are known to have died from the virus.
Kaduna state agriculture commissioner Lawal Yakawada said a team from Nigeria's National Veterinary Research Institute would "ensure that all the other chickens are killed, burned and buried".
He said the United States had pledged $20 million (€16.7m) to help fight the outbreak. Experts from the US Centers for Disease Control and Prevention would set up a laboratory in Nigeria, he added.
Kaduna and neighbouring Kano state have sent education teams to farming areas to warn of the danger of bird flu.
Since 1997, the H5N1 strain has spread across south Asia into southeastern Europe and has been blamed for the deaths of 88 people.
Experts fear Africa, with its underfunded health services and populations weakened by AIDS and malnutrition, may face an epidemic which would devastate poultry and encourage the virus to develop.
In the worst case scenario, if H5N1 mutates into a form transmissible between humans, it could kill tens of millions of people.
A South African veterinary institute is to conduct tests on samples from Kenya, Malawi and Sudan to help track the spread of bird flu.
Experts have advised Nigeria and any country which identifies a bird flu outbreak to limit the movement of poultry, quarantine infected farms and destroy sick birds. Nigeria has promised two billion naira (€13m) to do that and to compensate farmers for culled livestock.
Kenya has joined South Africa, Benin and Mauritania in banning poultry imports from Nigeria and ordered stepped up surveillance measures to prevent the spread of bird flu.