"The disease seems to have a regional dimension, with South Korea, Vietnam and Japan being affected in a short period of time," the United Nations (news - web sites) agency said in a statement.
"Possible links between these outbreaks must be investigated. If there are such links, the epidemiological reasons must be elucidated," the statement from Rome said.
An FAO official said the outbreak has taken an epidemic form in Vietnam, the hardest hit of the three countries.
Nearly two million chickens have been culled or killed by the flu in the southeast Asian nation, and the number culled in Japan and South Korea runs into the hundreds of thousands.
Fears over Asia's bird flu outbreak spiraled Wednesday as worst-hit Vietnam admitted that nearly 900,000 chickens possibly exposed to the deadly virus had been sold to the public
After days of denying it had a problem, Thailand became the latest flashpoint in the bird flu crisis with claims Thursday that a village boy is infected, while the World Health Organization (news - web sites) warned that the fast-spreading virus could mutate into a form deadlier to humans.
Bird flu has killed five people in Vietnam and infected millions of chickens across Asia. So far, fowl are believed to have directly infected people, but scientists worry about the possibility of human-to-human transmission.
"The more widespread it becomes, the greater the possibility that the virus could become altered and become more of a threat to the human population," said WHO spokesman Bob Dietz in Hanoi.
Thailand is among the world's top five poultry exporters and stocks in its multibillion-dollar industry dived as much as 7.1 percent on the Stock Exchange of Thailand.
China confirmed Tuesday that it found bird flu in dead ducks on a southern farm and was tracking "suspect" cases in two other provinces. Thailand said a young boy became the eighth victim of the disease in its deadliest outbreak since 1997.
In provinces like Guangdong -- where SARS first emerged -- people live in cramped conditions close to chickens and other farm animals, raising the possibility the virus may combine with human flu to produce a strain that could sweep through a world where people have no immunity to it.
The source of the sisters' infection has not been identified, but investigators have failed to find a specific event, such as contact with sick poultry, or an environmental source to explain the cases, WHO spokesman Bob Dietz said.
No other cases of people catching the virus from another person have been documented.