But the transmission "may have required very close human contact" and a sustained transmission is still unclear, according to the study, which will be published in the September edition of the medical journal Emerging Infectious Diseases.

The conclusion is based on statistical evidence of human-to-human transmission of the virus among seven deaths in a family of eight in northern Sumatra last year.

The study used a computerized disease-transmission model that took into account parameters such as the number of infected cases, the number of people potentially exposed and the viral- incubation period.

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It found a 37-year-old woman, who had been exposed to dead poultry and chicken feces, transmitted the virus to her 10-year-old cousin, who probably passed it to his father, as evidenced by genetic sequencing data.

"Although we've determined that human-to-human transmission had probably occurred, whether the virus is capable of sustained human-to-human transmission is not clear," the researchers wrote.

"This virus may have required very close human contact to be transmitted."

Hong Kong University microbiologist Malik Peiris said the new findings do not necessarily mean the virus is moving closer to human-to-human transmission.

"What's worrying is if there's sustained transmission, if it keeps going sequentially from human to human to human - but it hasn't done so, so far," he said.

Peiris noted that the finding of human-to-human transmission in Indonesia is not ground-breaking and should not add to unnecessary worries.

The six deaths from 18 human infection cases of bird flu in Hong Kong in 1997 were the world's first-known cases.

Another microbiologist, Lo Wing- lok, described the human-to-human transmission rate of 29 percent as "alarming," but he stressed the rate only applies to a certain part of Indonesia and under specific circumstances.

"The family of eight in Indonesia lived in a very crowded location, the people there have lower health awareness and have little access to medical care. All these do not apply in Hong Kong, " Lo said. "Hong Kong people are already highly aware and worried about it, and we already have easily accessible medical services."

Lo said Indonesia should share its flu virus samples with the international community and allow scientists to monitor viral evolution and develop vaccines.

A 28-year-old female chicken trader in Indonesia is suspected of having died of bird flu last week.

If confirmed, Indonesia will have 83 confirmed bird flu human fatalities - the highest for any nation.