The World Health Organization has announced a third confirmed case of H5N1 avian flu in Indonesia and warns of more to come.
But it also said Thursday there is still no evidence that the virus is spreading directly from person to person.
The latest patient, an 8-year-old boy, is being treated in a hospital along with a dozen more suspected cases. Another three suspected cases, all children, died this week.
"Given the experience of other H5N1-affected countries in Asia, the detection of further human cases in Indonesia or elsewhere would not be surprising," the WHO said in a statement.
The source of the illnesses is still under investigation. While there is no evidence of a mutant virus strain, or any other indication that the disease is spreading between people, WHO spokeswoman Maria Cheng told CNN that not every victim has had "obvious contact" with birds.
The victims' homes are scattered, but all are within a roughly 50-mile ring around Jakarta, she added.
Along with Indonesian investigators, half a dozen scientists from the WHO and one from the Centers for Disease Control and Prevention's field station in Thailand are helping to collect tissue samples and interviewing patients along with their close contacts, Cheng said.
Laboratory samples from the suspected cases have been sent to a WHO laboratory in Hong Kong, and results are expected by the middle of next week.
"Obviously, we'd like to get results as soon as possible," she said.
Cheng added that the surge in reported cases may be due to improved surveillance since July, when a 38-year-old man and his two young daughters became the first three Indonesians to die of avian flu. Last week, the WHO said it was unable to determine how they caught the illness, and that "testing and monitoring of more than 300 close contacts failed to detect any further cases."
Jakarta's zoo was closed last week after 19 birds were found infected with the virus. Some of the suspected human cases are workers at the zoo.
Earlier this week, Health Minister Siti Fadillah Supari said the situation could be considered an "epidemic," but took a step back on Wednesday.
"The extraordinary status of this contagious disease should not be considered as a frightening epidemic," he said, and promised an "all-out effort" to contain the virus.
In its most dramatic step, the Indonesian government said Wednesday it would start ordering mass killings of chickens on farms with infected birds, a measure that has long been urged by the WHO and other health experts. Until now, Indonesia has declined to order mass culls, saying it doesn't have the money to compensate its poultry farmers.
Indonesia has fired the country's chief of animal health control for allegedly failing to check the spread of the disease, The Associated Press reported.
Forty-four hospitals have been prepared to accept bird flu patients and the government said it could forcibly admit anyone with symptoms of the disease.
"We are observing 11 cases, including the one that died yesterday," I Nyoman Kandun, the head of disease control at Indonesia's health ministry, said Thursday. He was referring to a girl who died on Wednesday at Jakarta's infectious diseases hospital, Reuters reported.
Initial testing showed the girl did not die of bird flu, but further blood samples have been sent to a Hong Kong laboratory.
The H5N1 strain of bird flu has swept through poultry populations in large swaths of Asia since 2003, killing at least 63 people and resulting in the deaths of tens of millions of birds.
Most human cases have been linked to contact with sick birds. But the World Health Organization has warned the virus could mutate into a form that spreads easily among humans -- possibly triggering a global pandemic that could kill millions.
A top WHO official said the agency was prepared to begin distributing large-scale quantities of an antiviral drug to treat bird flu in humans "if and when a pandemic starts."
Dr. Shigeru Omi, director for WHO's Western Pacific region, told reporters at a WHO conference in New Caledonia that the U.N. agency was ready to open its stockpile of oseltamivir, an antiviral drug, to help avert a global pandemic of the disease.
WHO regards a pandemic as a multi-country outbreak of bird flu, in which the disease has been passed from human to human, CNN International reported.