The case involving up to seven family members, six of whom have died, has raised alarm around the world because authorities cannot rule out human-to-human transmission.

But the World Health Organisation and Indonesian health officials had been frustrated by the lack of evidence pointing to a source of the virus, usually infected poultry.

The WHO confirmed on Wednesday that five members of the family had contracted H5N1 and tests on a sixth were pending.

Officials had said earlier that on-the-spot testing of various animals living around Kubu Simbelang village in North Sumatra province had given negative results for avian influenza.

However, Agriculture Minister Anton Apriyantono told reporters on Thursday the pig samples from the village had been brought to a leading animal research centre on Java island, and scientists there found a positive result for bird flu.

"After we brought them to Bogor, the serology test found positive results. From 11 pig samples, 10 are positive. Reconfirmation testings are still underway," he said, but did not specify the H5N1 virus.

Bogor is a West Java city where a veterinarian institute is located.

Clusters of human infections are worrying because they indicate that the virus might be mutating into a form that is easily transmissible among humans. That, experts say, could spark a pandemic in which millions might die.

For the moment, the virus is mainly a disease in birds and is hard for humans to catch.

The minister's comments are also likely to concern health officials. Pigs can act as mixing vessels in which human and bird flu viruses can swap genes, leading to a strain that can easily infect people and pass from person to person.

At least 30 people have died of bird flu in Indonesia, the second highest toll of any country. More than half that number have died this year.

Not including the latest WHO confirmed cases in Indonesia, the disease has killed 115 people worldwide, the majority in east Asia, since reappearing in 2003. Virtually all the victims caught the disease from poultry.

The H5N1 virus is endemic in much of Indonesia.