The virus was also reported in Nigeria and Azerbaijan.
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.
Authorities asked citizens to avoid contact with fowl and to isolate domestic poultry from wild birds. Part of this former Soviet nation shares a short border with eastern Turkey, where four children died after becoming infected with bird flu.
International experts pressed Nigeria -- which reported Africa's first outbreak on Wednesday -- to do more to combat the disease.
Nabarro said one of the urgent needs is to establish how avian influenza reached west Africa. 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 and will try to match that sequence with viruses from birds in other countries affected by bird flu, he said.
The Paris-based World Organization for Animal Health and the Rome-based U.N Food and Agriculture Organization called on Nigerian authorities to close down poultry markets in affected states to prevent the virus from spreading.
Junaidu Maina, director of livestock at Nigeria's agriculture ministry, said the government would do "whatever needs to be done" to halt the disease's spread, but there was little progress reported on closing down poultry markets.
The WHO said anti-polio workers going door-to-door to immunize children in Nigeria starting Saturday could watch for unexplained deaths from pneumonia, a possible sign that someone has bird flu, said Bruce Aylward, coordinator of WHO's global polio eradication program.
The latest death in China was a 20-year-old female farmer from the county of Suining in the southern province of Hunan, the Health Ministry said on its Web site. It identified her only by the surname Long and said she had handled poultry.
Long fell ill Jan. 27 and died Feb. 4, the ministry said. It said laboratory tests confirmed she had the virulent H5N1 flu strain and the results were reported to the World Health Organization.
In Indonesia, officials citing local lab results said a woman from West Java province died of bird flu. The 23-year-old from Bekasi, a town just east of Indonesia's capital, Jakarta, died overnight after five days, said Ilham Patu, an official at the Sulianti Saroso Hospital.
Blood and swab samples from the victim have been sent to a WHO-accredited laboratory in Hong Kong for confirmation, he said.
The WHO says 88 people have died from bird flu 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, sparking a pandemic.
Associated Press reporter Daniel Balint-Kurti in Lagos, Nigeria, and Edith M. Lederer at the United Nations contributed to this report.