JAKARTA (Reuters) - Chickens are dying in unusually large numbers in a remote area in Indonesia where bird flu killed several members of a family, and experts say the first victim in the cluster was probably infected by a diseased chicken.

For weeks now, health experts have been trying to hunt down the source that introduced the H5N1 bird flu virus to the family in Kubu Sembilang village in north Sumatra, killing as many as seven of them.

The case has drawn immense interest because it is the largest known family cluster involving H5N1 and the World Health Organization (WHO) said this week that limited human-to-human transmission between members of the family might have occurred.

Tests done on samples from pigs, chickens and ducks in the area have been inconclusive and experts have long maintained that nothing can be ruled out. This is the first time that they have narrowed down the likely source to poultry.

"What we´re finding out the longer our team stays up in that area is that there are many, many outbreaks in chickens that always go unreported," Steven Bjorge, a WHO epidemiologist.

"Just in the past couple of weeks they have found a couple of outbreaks of chickens dying in various villages in that area ... that raises the very real possibility that people can come into contact with this virus."

Referring to the first victim in the cluster, a 37-year-old woman who died on May 4, Bjorge said: "The first case has to get it from somewhere. It has to be something environmental."

Asked if sick chickens were responsible for this index case, he said: "We think that it has to be that way."

The WHO has stressed that even if human-to-human transmission did occur, it was in a very limited way and the infection had not spread beyond the initial cluster.

In addition, scientific evidence had shown the virus had not mutated into one that can be easily passed among people -- a necessary precursor for a pandemic to start.

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Still, human-to-human transmission may have figured largely in the Sumatran cluster after the first woman was in all likelihood infected by chickens.

"We can´t rule it (human-to-human transmission) out in most of the cases, because they were all in very close contact in the very same environment. We can´t rule in out," Bjorge said.

Although Bjorge said it was up to Indonesia´s Agriculture Ministry to find out if the chickens around the Kubu Sembilang village were indeed dying of H5N1, he said Indonesia was awash with the virus.

"Basically, the virus is totally endemic in Indonesia, thoroughly entrenched in backyard chickens. That doesn´t mean they are dying every day in the same house. It jumps from place to place," he added.

Since re-emerging in Asia in late 2003, the virus has infected at least 218 people worldwide, killing 124 of them. Most of the cases occurred in places with outbreaks of the disease in poultry and victims were infected after direct contact with sick chickens.

Experts have stressed repeatedly the need to separate poultry from people -- advice many Asian communities ignore.