Nigerian agriculture ministry spokesman Tope Ajakaiye said tests on chicken carcasses had identified the H5N1 type of avian influenza, which can kill humans, in northern Nigerian sites more than 200 kilometres apart.
"Four farms have been cordoned off and quarantined; one in Kaduna, two near Kano and one in Plateau State," he said.
An official in Kaduna State said that two children living on another farm where birds were dying are to be tested for the human form of bird flu after they fell sick and started coughing up blood.
Previously, officials said the disease had only been identified at Sambawa Farm near Kaduna.
The announcement will increase fears that bird flu may be poised to spread rapidly around the country.
At Sambawa farm, 300 kilometres north of Abuja, police officers and agricultural officials moved in to cleanse a site where 45,000 chickens are now known to have died from the highly infectious virus.
Protective clothing
Armed officers in protective clothing set off to shoot 168 ostriches, while a bulldozer was brought in to bury the birds' cremated bodies.
"We have to kill and cremate all the birds from this farm, including ostriches, turkeys and geese. We will also burn their droppings," said Sa'idu Baba Chori, regional director of the state agriculture ministry.
"We have also received a complaint from a farmer in Kanta Road here that the doves, geese and chickens he is raising are dying rapidly and his two kids are sick. They are coughing blood," he said.
"We're now going there to take samples of the birds for laboratory analysis. The kids will also be examined to diagnose the nature of their ailment."
Agriculture ministry officials in the capital Abuja said experts from the
Food and Agriculture Organisation, World Health Organisation and the World Organisation for Animal Health were expected in Nigeria within 48 hours.
Mr Ajaikaye also said that the United States had pledged US$27 million to help Nigeria fight the outbreak.
Experts from the US Centres for Disease Control and Prevention would come from their Kenya base and set up a laboratory in Nigeria, and will supply about 2,000 protective suits for health and veterinary workers.
Since the H5N1 strain of bird flu was first detected in Hong Kong in 1997, it has spread across south Asia into southeastern Europe and has been blamed for the deaths of 88 people who came into contact with sick birds.
Fears of epidemic
Experts fear Africa, with its underfunded health services and populations weakened by AIDS and malnutrition, could now be facing a new epidemic, which would devastate poultry farming and encourage the virus to develop.
In the worst case scenario, if H5N1 mutates into a form which would be transmissible between humans, it could kill tens of millions of people.
A South African veterinary institute said it will conduct tests on samples from Kenya, Malawi and Sudan to help track the possible spread of bird flu.
"We will be testing samples collected for surveillance on the African continent," said Celia Abolnik, a researcher at the Onderstepoort Veterinary Institute, adding that the samples should arrive in the next two weeks.
Experts have advised Nigeria and any country which subsequently identifies a bird flu outbreak to limit the movement of poultry, quarantine infected farms and destroy sick birds.
Nigeria has promised two billion naira (A$20 million) to do just that and to compensate farmers for culled livestock, but health teams appeared to be slow at getting into place.
Auwalu Haruna, secretary of the Kano State poultry farmers' association warned that farmers and traders were slaughtering sick birds and rushing them to market for cheap sale "in a frantic effort to minimise losses."
Meanwhile Kenya joined South Africa, Benin and Mauritania in banning all poultry imports from Nigeria and ordered stepped up surveillance measures aimed at preventing the spread of bird flu.