Beijing ? Health authorities in China and Indonesia on Friday each reported a woman killed by a deadly strain of bird flu. Azerbaijan became the latest country to report an outbreak among fowl, and Nigeria said the virus had spread there, too.

The spread of bird flu has increased the chance that the virus will mutate and cause disease in humans and a possible pandemic, said Dr. David Nabarro, the U.N. bird flu chief. He said there is no evidence yet of any change in the bird flu virus.

“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. “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.”

If the death in Indonesia is confirmed by the World Health Organization, it would be the country’s 17th. The Chinese death would be the eighth there, if confirmed. China on Wednesday reported a woman died of the H5N1 strain in an area with no reported outbreaks in poultry.

Azerbaijan’s Health Ministry said a British laboratory had confirmed the H5N1 strain in wild ducks and swans found on the Absheron Peninsula, which juts into the Caspian Sea and includes the capital, Baku.

International experts pressed Nigeria – which reported Africa’s first outbreak on Wednesday – to do more to combat the disease, and a top agriculture official said the virus had been reported on a total of five farms in the northern part of the country. No human infections have been reported.

Nabarro said one of the urgent needs is to establish how avian influenza reached west Africa. U.N. experts have received the genetic sequence of virus samples taken from the farm in Kaduna where the H5N1 strain of bird flu was discovered and will try to match that sequence with viruses from birds in other countries affected by bird flu, he said.