BANGKOK (Reuters) - Thailand said on Tuesday it had found its first known probable case of a human being infecting another with bird flu, but insisted it was an isolated incident that posed little risk to the greater population.

The government said a 26-year-old woman who died on Sept. 20 could have caught the H5N1 virus in the village where her daughter lived but probably was infected by the 11-year-old girl while looking after her in hospital.

"It would have been due to close and prolonged face-to-face exposure," the government statement said, adding that no health workers at the hospital had fallen ill.

The World Health Organization and the U.S. Centers for Disease Control and Prevention had been part of the investigation of the case from the beginning and agreed it "would not pose a significant public health risk," the statement said.

The mother´s death took to 10 the number of Thais killed by the H5N1 virus. In Vietnam, 20 people have died of the disease.

Experts have long feared the H5N1 bird flu virus, which swept through much of Asia early this year, could mutate into a form that could be passed from person to person and set off a pandemic like the one in 1918, which killed 20 million people.

But, they say, the H5N1 virus would have to go through an animal -- most likely a pig, although cats can also get it -- capable of harboring the human influenza virus with which it could merge to forge a virus that could trigger a pandemic.

The experts agreed "there is no evidence to suggest that the virus has mutated," the statement said.

Thai Prime Minister Thaksin Shinawatra called an emergency meeting of provincial governors on Wednesday to beef up measures against the disease, a government spokesman said.

"FLUKE" DISCOVERY

CDC expert Scott F. Dowell said the Thai case was a "fluke" in that the mother and daughter lived apart, whereas in families living under the same roof it was impossible to tell if infected people caught bird flu from chickens or from each other.

Suspicions of people-to-people transmission in Hong Kong in 1997, when the H5N1 virus first appeared, and in Vietnam this year could not be substantiated for that reason, he said.

"Because the mother lived in a separate city, it was much easier to be confident that the likely mode of transmission or the probable mode of transmission was person to person," he said.

"What appears to have happened so far is a well-documented episode of transmission in a family cluster after prolonged, close contacts. It doesn磘 necessarily tell us that anything has changed about the virus and about the way it transmitted.

The daughter died on Sept. 8 in the north-central province of Khampaeng Phet to which the mother had traveled from her job on learning the girl was ill.

The mother then went back to work and died soon after. An aunt, with whom the girl lived while her mother was away working, was confirmed on Monday to have the H5N1 virus, but was reported to be recovering.

The girl and the aunt both had contact with infected chickens in the village. Fowl were still dying in the village when he visited it at the weekend, said Charal Trinvuthipong, head of the Department of Disease Control.

International organizations are deeply worried about the disease, probably carried across Asia by migrating water fowl that infected domesticated birds and forcing the slaughter of tens of millions of fowl in an effort to prevent its spread.

Thailand and Vietnam, the worst affected, have said several times they had control of the epidemic. However, it broke out again in July, proving how hard it is to stamp out the virus.

"The avian influenza epidemic in Asia is a crisis of global importance," the U.N. Food and Agriculture Organization and the world animal health organization OIE said in a statement.