It comes after the strain deadly to humans was detected on a farm in Kaduna in northern Nigeria.Officials are investigating whether poultry in other states have also died from the virus. Dr Nabarro said the WHO was anticipating further outbreaks in other parts of Africa.
"If it's in Nigeria it might also be in other countries that are less well-equipped." He said governments and ordinary people would have to take "very, very strong precautions" to protect themselves and stop the disease spreading.
"We've got to have all countries, particularly in West Africa, being very vigilant for bird die-offs, which are the indicator of bird flu being in the population."
Experts have been sent to the commercial chicken farm in Jaji, in Kaduna state, where bird flu was found. 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.
Dr Nabarro, who is leading the UN's response to bird flu, said reports from Nigeria's ministry of agriculture suggested bird flu may also have been found in the northern state of Kano and further south in Jos.
The BBC's Alex Last in Lagos, Nigeria, says an outbreak of bird flu could have devastating consequences in a country where millions of people rear chickens as a basic source of income.
Experts have long feared that if H5N1 reached Africa, it could quickly take hold and spread out of control. Dr Alex Thiermann of the Paris-based World Organisation for Animal Health told the BBC that Africa's "veterinary infrastructures are very weak".
"And it is essential for the containment of this disease to have the ability to early detect and take rapid action. And therefore we feel that Nigeria and the other countries that are at risk need help very quickly."
Sold as meat
Nigeria says it will cull all infected birds and compensate farmers. But a northern Nigerian farmer told the BBC News website that people fear they will not be paid.
"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.
It is thought bird flu may have been carried to Nigeria by migrating birds or the smuggling of infected chickens from abroad.
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, it could become a global pandemic, killing millions.
Source: BBCLast Mod: 00 0000, 00:00