UNITED NATIONS — The spread of bird flu from Asia to eastern Europe and now west Africa has increased the chance the virus will mutate and set off a pandemic, the U.N. bird flu chief said.

Dr. David Nabarro said there is no evidence yet of any change in the virus, which has killed at least 88 people since 2003.

Almost all the deaths have been linked to contact with infected poultry, but experts fear the virus could mutate into a form that spreads easily among humans, setting off a pandemic.

"Unfortunately, we cannot tell when the mutation might happen, or where it might happen, or how unpleasant the mutant virus will turn out to be," he said in an interview. "Nevertheless, we must remain on high alert for the possibility of sustained human-to-human virus transmission and of a pandemic starting at any time."

Nabarro said the arrival of bird flu in Nigeria should be "a strong wake-up call" to all countries to ensure that their veterinary services are on alert and report any instances of birds or poultry dying, and that health services quickly identify unexpected clusters of unexpected disease that could represent the start of a pandemic.

"We have got bird flu now in southeast Asia, central Asia, eastern Europe, and west Africa," he said. "Compared with eight months ago, this is a major extension of the avian influenza epidemic."

Nabarro said control measures put in place by countries have helped to contain the spread but bird flu is still expanding across the world "putting at risk the health of people who are living intimately with poultry and also adding to the overall load of the H5N1 virus."

He said it is the increase in the quantity of the virus in the world today that has boosted the overall chance of mutations, including a mutation that could cause a disease which could then spread through the human population.

"That's why we get so concerned about the spread of the virus, because we want to do everything we can to reduce the opportunity for mutation," Nabarro said.

He said one of the urgent needs is to establish how avian influenza reached west Africa.

"The likely means is by migrating wild birds traveling from north to south, and one of the main migratory routes passes from Siberia through the Black Sea area, including Crimea and on to west Africa," Nabarro said. "The alternative is that the virus arrived in birds that are being traded — and if that is the case they would have been smuggled as Nigeria had banned import of birds from avian influenza affected areas during the last two years."

U.N. experts have just received the genetic sequence of virus samples taken from the farm in Kaduna where the H5N1 strain of bird flu was discovered, he said.

Over the next few days, he said, the World Organization for Animal Health and the U.N. Food and Agriculture Organization will try to match that sequence with the genetic sequence of viruses from birds in other countries affected by bird flu, he said.