Indonesian health officials have expressed worries about the likelihood of human-to-human bird flu virus transmission due to the growing cases of bird flu clusters, a newspaper said Friday.

Cluster cases, which is growing rapidly in the country, are defined as several members of a household or neighborhood falling sick at the same time.

"We can't guess when the spread of the virus among humans will occur because it will need a thorough examination of the source of the virus from each patient in a cluster," the spokesman and head of the bird flu surveillance unit at Jakarta's Sulianti Saroso Hospital, Ilham Patu, was quoted by The Jakarta Post as saying.

"But the fact that we have more and more cases of bird flu clusters shows that we are very close to having one."

Health Minister Siti Fadillah Supari said Wednesday the country had the highest number of bird flu cluster fatalities in the world, with six reported so far. Indonesia also has the world's highest mortality percentage, with 18 fatalities of the 26 people who have tested positive for the H5N1 virus since July last year.

The minister said the virus appeared to be more virulent and there was a growing probability of human-to-human transmission.

The World Health Organization said it had not found any evidence of changing characteristics of the virus' transmission here.

"We still have Indonesia at level three. Although it means the bird flu virus still comes from animal (hosts), we don't rule out the imminent possibility that limited viral transmission between humans could occur," WHO public relations officer Sari Setiogi said.

Source: Xinhua