Until recently the bird flu virus has been considered largely an Asian problem, but experts are now waking up to the fact that it could be a global threat unless measures are taken to contain the disease.

Chickens sit in cages at a Changji farm, in China's western Xinjiang region

Concern is growing that the H5N1 virus, which causes bird flu, may mutate into a form easily passed between humans, causing a pandemic that could kill millions of people.

The bird flu virus has swept across Asia since 2003, causing the deaths of more than 60 people in four countries and decimating the region's poultry industry. The latest deaths occurred in the Indonesian capital of Jakarta, where four people died in the last two months.

Although most of the victims caught the disease from close contact with infected poultry, the infection source for those who died in Jakarta has yet to be determined.

Indonesian Agriculture Ministry officials give bird flu vaccine to a chicken at during a door to door search for poultry in Jakarta

Last week a Jakarta zoo was closed when several species of birds, including eagles and peacocks, tested positive for the virus.

Steven Bjorge, an epidemiologist for the World Health Organization in Jakarta says this calls into question the assumption that people are being infected by poultry alone.

"Certainly the information from the zoo raises a lot of questions about whether or not the true exposure in Jakarta is something other than chickens and poultry, maybe it's some other species of birds that are spreading the virus," he said.

Mr. Bjorge also says the H5N1 virus may be changing in birds into one that can more easily infect people, as may have been the case for the 1918 Spanish flu, which killed tens of millions of people globally. "The 1918 virus was a bird virus through and through - and the current thinking is that it did modify itself to be more easily transmitted to humans," he said.

But although health experts do not know exactly how the virus is spread, they say governments can still take effective preventative measures.

Diderik de Vleeschauwer, Asia-Pacific spokesman for the U.N.'s Food and Agriculture Organization says the Indonesian government must improve procedures to contain the recent outbreak.

"We feel the bird flu has become endemic in Indonesia - and it indeed seems that it is continuing to spread," he said. "That is definitely a worrying situation and it is necessary for the government to improve on its virus control policies."

Indonesia announced earlier this week emergency measures to contain bird flu, including a mass cull of chickens and the forcible hospitalization of people suspected of having the disease.

The recent H5N1 outbreak in Indonesia and the discovery last month of the virus in poultry in Russia and Kazakhstan have prompted the United States to launch an international partnership on avian flu and call for early warning pacts with Southeast Asian governments.

Bob Dietz, the WHO's Asia-Pacific spokesman says Western nations are increasingly aware of the need to help Asia fight the virus. "Western nations now realize that if they contain this in Southeast Asia, in fact they might minimize the jeopardy to their own citizens," he said. "They are now starting to put money into helping these countries develop surveillance systems, helping these countries to prepare to contain this disease within their own countries."

But Mr. Dietz is still concerned about the ability of Asian countries to contain the virus. "Most of Asia, where we're seeing a lot of this H5N1 activity is not very well prepared, mainly because they don't have the resources to meet the challenge," he said.

Health experts from the United States, Australia, and Canada have begun arriving in Jakarta to help Indonesia deal with its growing bird flu problem.