The Public Health Ministry will dispatch epidemiologists to Indonesia to join a World Health Organisation (WHO) team observing the latest report of limited human-to-human transmission of bird flu.

Dr Kamnuan Uengchusak, director of the Communicable Diseases Control office, said the WHO had invited Thai experts to join in studying the case in which seven members of a family died from the disease.

The latest announcement by the WHO said the seven fatal cases in North Sumatra, the largest cluster of infections in Indonesia, had heightened fears that the virus had passed between people.

The WHO said that although human-to-human transmission could not be ruled out, the search for a possible alternative source of exposure was continuing.

It said there was no evidence the virus had mutated into an easily transmissible form. The WHO's statement, which was posted on its website, said: "All confirmed cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness.

"To date, the investigation has found no evidence of spread within the general community and no evidence that efficient human-to-human transmission has occurred."

The WHO said genetic sequencing revealed no evidence of significant mutations of the virus. Scientists fear a global pandemic if the virus mutates and becomes easily transmissible between humans.

More than 120 people worldwide have died from bird flu since it re-emerged as a threat in 2003 - most of them in Asia.

Indonesia has had more bird-flu deaths than any other country this year.

It has the world's second highest number of fatalities since 2003, after Vietnam.

Nine Indonesians who were infected have survived.