Nigeria will kill all birds at any farm where suspicious deaths have occurred in a bid to contain Africa's first outbreak of bird flu, a minister says.
The bird flu outbreak could be devastating for many Nigerians
Bird flu has been confirmed at one farm in the northern state of Kaduna. Some 45,000 chickens died on a farm owned by Sports Minister Saidu Balarabe Sambawa.
Tests are being carried out on dead poultry at two more farms.
Chickens started dying four weeks ago, leading to fears that the H5N1 bird flu strain may have already spread widely.
Quarantines and other restrictions are only now being imposed on farms near where the chickens have died.
Thousands more chickens have died in the two neighbouring states of Kano and Plateau. The cause of these deaths has not yet been determined but international experts are heading for all three areas.
Dr David Nabarro of the World Health Organization (WHO) told the BBC the virus "might be quite widespread".
"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.
A team of Nigerian experts is heading for Mr Sambawa's farm to cull the 200 ostriches which are still alive.
The BBC's Adamu Yusuf at the farm says a quarantine has been imposed and all workers have been sent home.
Mr Sambawa has told the BBC's Adamu Yusuf that he suspects the outbreak is down to "sabotage", possibly by disgruntled former workers at the farm.
Mr Sambawa was speaking by telephone from the African Cup of Nations in Egypt, where he saw Nigeria lose in the semi-finals on Tuesday.
Agriculture Minister Adamu Bello, who announced the mass cull, has suggested illegal poultry imports may be behind the outbreak.
The disease may also have been spread by migrating birds.
Rushing to market
Correspondents say an outbreak of bird flu could have devastating consequences in a country where millions of people rear chickens as a basic source of income.
A northern Nigerian farmer has told the BBC News website that farmers are rushing to sell dead chickens in markets before restrictions are imposed.
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".
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.