JAKARTA, Indonesia — A woman died of bird flu on the western outskirts of Indonesia's capital Tuesday, raising the country's death toll from the virus to 57, a senior health ministry official said.

The 35-year-old woman from the city of Tangerang died after four days of hospitalization, said Nyoman Kandun, adding the source of infection was still under investigation.

The H5N1 virus has killed at least 153 people worldwide since it began ravaging Asian poultry stocks in late 2003, according to the World Health Organization, with well over a third of the human deaths in Indonesia.

The WHO has not yet confirmed the last two Indonesian cases.

Most of those killed have been infected by domestic fowl, but WHO fears the virus could mutate into a form that easily spreads among humans, sparking a pandemic with the potential to kill millions.

Indonesia, the world's fourth most populous country and home to millions of backyard chickens, is considered a potential hotspot for that to happen.

The government came under fire for moving slowly to stamp out the virus when it first appeared in chickens and ducks, but has worked hard in recent months to raise public awareness about the dangers of bird flu.

It has also increased vaccinations and slaughtering of birds in infected areas.