Indonesia has taken immediate action to try to contain an outbreak of the bird flu virus that has killed four people in the nation's capital since July. International health experts have begun arriving in Indonesia to help with the growing problem.

The Indonesian health minister warns the recent outbreak of bird flu could easily turn into an epidemic as agricultural officials announced plans for a massive cull of poultry in infected areas.

Health experts are concerned because they cannot identify the source of the H5N1 virus that has caused at least four deaths in recent weeks.

World Health Organization spokesman Bob Dietz says there is concern the virus could spread among humans.

"The threat, the very, very, very grave threat is that sooner or later it will evolve into that sort of virus that can be exchanged between human beings. That means it could tear through the global population very quickly," he said.

Six suspected cases of bird flu remain under observation in a Jakarta hospital, while a seventh, a five-year-old child, died Wednesday.

Most of the at least 60 people who have died from the virus in four Asian countries since late 2003 became infected after close contact with birds.

The WHO's Indonesia representative, Georg Petersen, says with 12 million people in Jakarta living near infected birds more people could catch the virus.

"The virus is basically circulating in the bird population and we have a situation with very high population density too in the same areas, so the chances are more infections in humans is slowly growing," he said.

WHO spokesman Mr. Dietz also says human-to-human transmission may have already occurred, but in a limited way.

"This might have happened. But we have always been reluctant to say categorically this has indeed happened. What might be happening is inefficient transmission of the virus between human beings in some very specialized, localized situations," he said.

Health experts from Australia, the United States and Canada have begun to arrive in Indonesia to assess the situation.