The U.N. health agency is looking closely at possible limited human-to-human transmission of bird flu between members of an Indonesian family, but said there was no evidence indicating the virus had mutated or that it had spread beyond the relatives.

"We're not surprised that there is possible human-to-human transmission," said Steven Bjorge, a World Health Organization epidemiologist in Jakarta, Indonesia. "The thing we're looking for is whether it's sustained beyond the immediate cluster."

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Six of the seven people in the family from northern Sumatra who have caught the deadly disease have died, the most recent on Monday. It is one of the largest human clusters ever reported.

Bjorge, who is the team leader at the village in Kubu Sembelang, said none of the poultry in the area tested positive for the H5N1 bird flu virus, which has led a team of international experts to explore whether the virus spread among the relatives.

He warned, however, that such isolated cases of very limited human-to-human transmission have been documented — including a case in Thailand involving a mother and child — and that it does not mean a pandemic flu strain has emerged.

Bjorge said the virus has not altered its form in any way and is genetically the same as the virus found circulating in the area earlier.

"That, for me, is the most significant piece of evidence," he said. "Despite some weeks now in following up, we cannot find any evidence of any other cases beyond this cluster. If either of those two things changed, then I would be talking very differently."

Bjorge said some samples have been taken from villagers, but cooperation has been limited. If anyone outside the family is found to have even mild flu symptoms, he said they would be quarantined and given the anti-bird flu drug Tamiflu.

Earlier in a statement on its Web site, the WHO in Geneva said it was still investigating the cluster, but experts were looking closely at the possibility of limited human-to-human transmission. Scientists from the WHO and the U.S. Centers for Disease Control and Prevention are investigating.

"All confirmed cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness," the WHO statement said. "Although human-to-human transmission cannot be ruled out, the search for a possible alternative source of exposure is continuing."

Health experts are concerned that if the virus mutates to a form that is easily transmissible between people, it could lead to a pandemic. So far, most human cases have been traced to contact with infected poultry.

Bird flu has killed 123 people worldwide, nearly a quarter of them in Indonesia.

The spread of bird flu from one person to another is very rare, and it has always faded out after that. The start of a flu pandemic would require the virus to be easily spread among people.