The measures announced by Nigeria's government to contain the spread of bird flu are not working, the UN says.
Poultry are still being moved around despite an official quarantine, a senior official at the Food and Agriculture Organisation says.
Juan Lubroth also said the deadly H5N1 strain may have been found in nearby Niger - which denied the reports.
No human cases of bird flu have been found in Africa, although Nigeria chicken farmers are being screened.
Thousands of chickens have been dying in Nigeria for more than a month.
All four confirmed cases of bird flu have been linked to a single farm in northern Nigeria, said Agriculture Minister Adamu Bello.
None of the workers on the farm have shown any signs of illness.
The official in charge of the clean-up campaign has said that 40 farms around Kano city had reported suspicious bird deaths.
"The disease is spreading fast," he told the AFP news agency.
There have been numerous reports of chicken farmers rushing to sell their birds before officials cull them. They say the compensation offered by the government is too low.
"There is a restriction of movement for animals out of Kano and Kaduna states but on the ground it does not really seem to be holding," Mr Lubroth said.
But a Niger government spokesman denied that the H5N1 strain had been found across the border, with two dead roosters reportedly being tested.
"Until there is proof to the contrary, and we repeat this, there is no avian flu outbreak in Niger," said Mohamed ben Omar.
Public awareness
Experts with protective equipment are being flown into Nigeria from abroad to help local officials contain the spread of the disease.
Neighbouring countries have banned chicken imports from Nigeria.
The World Health Organization (WHO) has called for a massive public education campaign to stop bird flu from spreading to humans in the country.
It is not clear how the virus has spread there. It was first found in South-East Asia and more recently Turkey and Russia.
Some experts blame illegal poultry imports; others migrating birds.
More than 80 people have died of H5N1 bird flu since the disease's resurgence in December 2003 - most of them in South-East 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, it could become a global pandemic, killing millions.
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