JAKARTA, Indonesia - The U.N. health agency is investigating whether bird flu was passed from human-to-human among members of an Indonesian family, but said Wednesday there is no evidence the virus has mutated or spread beyond the family.

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

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

Person-to-person transmission of bird flu is rare, but it is one of the biggest concerns about the disease. Scientists say that if the virus mutates into a form easily passed between people, it could cause a pandemic that could kill millions.

Bjorge, the WHO team leader at the village in Kubu Sembelang, said none of the poultry in the area had tested positive for the H5N1 virus, leading international experts to explore whether the virus spread among the family members.

Isolated cases of very limited human-to-human transmission have been documented - including one in Thailand involving a mother and child - but such cases do not mean a pandemic flu strain has emerged.

Bjorge said the virus found in Kubu Sembelang is genetically the same as the one 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."

Indonesia and other countries like Vietnam have experienced similar family clusters in the past, but none as large as the Indonesian one.

"We are worried for two reasons," said Peter Cordingley, spokesman for the WHO regional office for the Western Pacific in Manila. "One is we've never seen a cluster this size before. And the second reason is that we have a team down there, they are examining what is going on and they can't find an animal source of this infection, and that worries us."

Bjorge said the woman first believed to be infected by the virus worked as a vegetable vendor in a market where live poultry was sold. Experts are trying to determine if that's where she became infected. The woman, who died May 4, was never tested for the H5N1 virus, but WHO considers her part of the family cluster. The woman's 25-year-old brother is the only family member still living after being infected.

"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 said in a statement on its Web site.

Bjorge said some samples have been taken from villagers, but that local authorities have resisted working with outside health experts. WHO has enlisted local villagers to help monitor the village for anyone experiencing flu-like symptoms.

If anyone is found to have even mild symptoms, they will be quarantined and given the anti-bird flu drug Tamiflu, he said.

Bird flu has killed 124 people worldwide, more than a quarter of them in Indonesia. So far, most human cases have been traced to contact with infected poultry.

"Indonesia at the moment has very, very serious problems with this virus," Cordingley said. "(It's had) lots of cases this year and it doesn't seem to be any closer to control."