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

Health experts have been trying to find the source that introduced the H5N1 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.

The WHO is not considering raising its global bird flu alert level, although its investigation into the family cluster in Indonesia continues, a senior WHO official said on Friday.

Paul Gully, senior adviser to the WHO´s top bird flu official Margaret Chan, told Reuters in an interview in Geneva: "Our feeling now is there is nothing new that has happened which would make us want to consider moving to level four."

The United Nations agency currently rates the level of pandemic threat at three on a scale of six, meaning some very limited human-to-human transmission has occurred. Level four would signal evidence of increased human-to-human transmission.

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 fear the virus could spark a global pandemic in which millions could die if it mutates into a form that passes easily from person to person.

Health experts have now asked 54 people in the village to quarantine themselves, up from 33 earlier, but none are reported to have any bird flu symptoms, the WHO said on Friday.

TRACKING THE SOURCE

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," said 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."

Experts say genetic analysis of the viruses taken from the victims do not show any of the obvious changes that would allow H5N1 to pass easily among people.

The virus has spread rapidly in recent months to countries in the Middle East, Africa and Europe.

Romania has reported more than 70 outbreaks of avian flu in birds over the past two weeks, mostly in the central region of Transylvania, a month after the strain was said to have been eradicated in the east European state.

The latest outbreaks originated at a poultry farm in the county of Brasov, some 170 km (100 miles) north of Bucharest, from where live chickens were sold to peasants across the country without health certificates, the government says.

(Additional reporting by Stephanie Nebehay in Geneva and Radu Marinas in Bucharest)