World health chiefs said today that they were increasingly concerned about the bird flu deaths of six Indonesians from one family, raising fears of the first human-to-human transmission of the deadly H5N1 virus.

Seven members of an Indonesian family in the village of Kubu Sembelang, northern Sumatra, have been infected with bird flu, of whom six have died, despite not coming into contact with diseased birds.

Last night, the Word Health Organisation said human transmission was a possible cause of the cluster, but said there was no evidence of a wider spreading of the virus.

"This is the most significant development so far in terms of public health," Peter Cordingley, spokesman for the West Pacific region of the WHO, told Reuters Television in Manila today.

"We have never had a cluster as large as this. We have not had in the past what we have here, which is no explanation as to how these people became infected."

"We can’t find sick animals in this community and that worries us," he admitted.

The possible mutation of the H5N1 virus to a form more easily transmitted between humans is the greatest fear of scientists and public health officials.

Although a mutation would make the virus less deadly, it would still pose a danger to millions of lives. The WHO stressed last night that no mutation had occurred in the virus found in the Indonesian family.

"All confirmed cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness," said the WHO statement.

"Although human-to-human transmission cannot be ruled out, the search for a possible alternative source of exposure is continuing.

"To date, the investigation has found no evidence of spread within the general community and no evidence that efficient human-to-human transmission has occurred."

The initial infection is believed to have been caused by an eighth member of the family, a vegetable seller, who died on May 4 of a respiratory disease. Health officials believe she may have contracted the virus from live birds at the market where she worked.

WHO officials in the village believe that the likeliest moment of transmission came on April 29, when the woman first infected by the virus spent the night coughing in close proximity to her brother and two children, all of whom contracted the disease shortly afterwards. The rest of the family lived in adjacent buildings.

The most recent death, of a 32-year-old man, came after he spent several days nursing his son. He died on Monday prompting the WHO to make its first statement on the case, which has attracted growing concern in recent days in Indonesia and across Asia, where financial markets trembled at last night's news.

Bird flu has killed 124 people in ten countries since it re-emerged in Asia in 2003. Nearly every human case of the disease has been the result of close contact with dying birds. Human transmission, although unproven, is believed to have taken place in some families in Hong Kong and Vietnam.

News of the Indonesian case comes as health officials in Iran investigate a family of five H5N1 cases in the northwestern city of Kermanshah.

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