The World Health Organisation said yesterday it was "alarmed" by a cluster of bird flu cases in Indonesia that showed every sign of having been spread from one human to another.

Seven members of an extended family in north Sumatra fell ill with bird flu and six of them have died. The alarm has been caused by the fact that most of the family appeared to have caught the infection not from chickens, but from close contact with each other.

Flu experts said a pandemic would become a real danger when the avian flu virus known as H5N1 becomes easily transmissible between humans.

Until now there have been only a couple of isolated incidents where one person is believed to have infected another. One of those was a woman from Bangkok, who returned to her village to nurse her child who was sick with avian flu. Although the mother had not had any contact with chickens, both died.

Samples of the virus in the bloodstream of the Sumatran victims have not so far shown any mutation of the H5N1 virus which would have to happen for it to become easily transmissible between humans. But the scale of this cluster, and the possibility of further cases, has raised serious concerns. "We are alarmed by this cluster," said Maria Cheng of the WHO. "It is the largest cluster we have had that has been identified for possible human to human transmission. The hypothesis is that these people were at very close quarters and were taking care of each other and falling ill."

WHO officials, including senior epidemiologists, have flown to Kubu Sembelang village in Karo district to try to establish the chain of events, which appears to have begun when a 37-year-old woman, who held a barbecue for her extended family, fell ill. On the night of the party, on April 29, the woman, her two sons and her brother, 25, spent the night in one small room, where she was reported to have been coughing and showing other flu-like symptoms. The woman and the boys later died. The brother became ill, but survived.

Other members of the family lived in adjacent houses in the village, and investigators are trying to find out whether they caught the virus from looking after those who fell sick. The latest to die, on Monday, was another brother of the woman, aged 32. He is thought to have caught bird flu from his own son, 10, who has also died.

More than 30 people in the village have been quarantined and are being treated with the antiviral drug Tamiflu. Officials are seeking anyone who has had contact with those who became sick. They say that although transmission appeared to have been from one person to another, there was no evidence it was "efficient".

Indonesia has had the biggest number of confirmed bird flu cases in humans - more than 30 - and they continue to occur, unlike in Thailand and Vietnam, where outbreaks have declined.

This month Egypt reported its 14th human case, in a 75-year-old woman who died, Djibouti its first, in a two-year-old girl, and China its 18th, in an eight-year-old girl.