Preliminary tests have found that bird flu killed two more siblings in Indonesia, officials said Friday, as the country grapples with a separate outbreak involving the largest family cluster ever reported.

A youth stands with her dog outside her house as a chicken walks past by in Kubu Simbelang vilage where seven of a family of eight have been decimated by bird flu deaths, in North Sumatra, Indonesia, Thursday, May 25, 2006. The members of the family probably passed the disease among themselves, but health officials said there was no evidence the virus had mutated and decided against raising the pandemic alert level. [AP]

Local 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 further 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.

The newest cases come as Indonesia is struggling with a different family cluster in northern Sumatra where six of seven family members died of bird flu, the most recent on Monday. An eighth family member who died was buried before tests could be done, but she was also considered to be among those infected with bird flu.

WHO officials have not been able to link the family members to contact with infected birds, and have said it's possible limited human-to-human transmission may have occurred. Similar isolated cases of transmission among humans is believed to have occurred in four or five other family clusters, said WHO spokesman Dick Thompson. But the Indonesia case is the largest ever reported.

However, the WHO has stressed the virus has not mutated in any way and has shown no signs of spreading outside the family _ all blood relatives who had very close contact with each other.

A team of international health experts and villagers is closely monitoring the area where the family lived in northern Sumatra to ensure no one else experiences flu-like symptoms.

About 30 people in the village of Kubu Sembelang 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 may have had contact with sick or dead chickens. She also worked at a market where chickens were sold and may have used chicken feces as a garden fertilizer, WHO officials have said.