"All confirmed cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness," said a WHO statement. "Although human-to-human transmission cannot be ruled out, the search for a possible alternative source of exposure is continuing."
The agency's alert level remained Wednesday at 3, where it has been for months. That means there is "no or very limited human-to-human transmission."
WHO spokeswoman Maria Cheng said it was unlikely the agency would raise the alert level in the immediate future.
"We haven't seen evidence from Indonesia that the disease is passing easily from human to human," Cheng told The Associated Press.
She said WHO had considered convening a meeting of experts to debate whether to raise the alert level, but had decided that the current situation did not merit that step.
The agency has suspected that in rare cases bird flu may have passed from one person to another, but it usually has been caught by people from chickens and other poultry.
WHO said that testing indicated there had been no significant mutations in the virus. Experts have feared that a mutation of the virus into a strain that could easily pass among humans could set off a deadly flu pandemic.
According to the WHO, 218 people have been confirmed to have been infected with bird flu since 2003, and 124 of them have died.
The agency said the Indonesian Health Ministry had confirmed a man who died May 22 had been infected with the deadly H5N1 strain of bird flu, the AP reports.
He was the seventh member of an extended family confirmed to have become infected. An eighth person in the family, who died of similar symptoms May 4, was buried before tissue samples could be taken, so the cause of death could not be determined, but she is assumed to be part of the cluster, WHO said.
The family lives in the Kubu Sembelang village, Karo District, of North Sumatra.
It said the investigation is continuing, but that preliminary findings indicate that three of the confirmed cases spent the night of April 29 in a small room with the first woman infected and that she was coughing frequently.
That group included the woman's two sons and a second brother, who is the sole surviving case among infected members of this family, WHO said. Other infected family members lived in adjacent homes.