Preliminary tests have found that bird flu killed two more siblings in Indonesia, officials said Friday, as they searched for the origin of a separate outbreak that has killed at least six members of the same family.

Initial tests found that a brother and sister from West Java who died earlier this week were infected by the H5N1 virus, said Nyoman Kandun, head of the Health Ministry's office of communicable disease control.

The tests will be sent to a World Health Organization laboratory for confirmation. WHO officials so far have confirmed 33 human deaths from bird flu in Indonesia, out of 124 worldwide.

The latest victims, an 18-year-old boy and his 10-year-old sister, died Tuesday in the state-run Hasan Sidikin hospital in Bandung, the capital city of West Java, said Achmad, an official at the ministry's special task force post for bird flu, who uses only one name. They died within hours of each other less than a day after arriving at the hospital, he said.

In northern Sumatra, WHO officials say an outbreak involving at least seven members of the same family, six of whom died, appears to be an isolated case of limited human-to-human trasmission.

However the WHO says the virus has shown no signs of having mutated or of spreading outside the family — all blood relatives who had close contact with each other.

Health experts are concerned that if the virus mutates to a form that is easily transmissible between people, it could lead to a pandemic.

Still, international health experts are closely monitoring other people in the village of Kubu Sembelang for flu-like symptoms.

About 30 have been asked to stay inside their homes and avoid close contact as a precautionary measure, Thompson said.

An eighth relative was dead and buried before tests could be conducted.

The case is not the first involving a family cluster but it is the largest one, said WHO spokesman Dick Thompson. He said the agency has documented four or five cases in which limited transmission among humans is believed to have occurred among relatives.

International health experts continue to monitor other people in the village of Kubu Sembelang for flu-like symptoms.

About 30 have been asked to stay inside their homes and avoid close contact as a precautionary measure, Thompson said.

Experts also are exploring whether the first woman sickened in the family got the disease from contact with sick chickens at the market where she worked or through chicken feces she used to fertilize her garden.