Thai and international health officials were due to hold an emergency meeting on Monday to review a case in which there is a "very remote possibility" bird flu was passed from human to human, a senior WHO official said.
"It was a preliminary suspicion," Dr Kumara Rai, acting head of the World Health Organization in Thailand, said of the possibility of human transfer of the disease, which has killed 20 Vietnamese and nine Thais this year.
However, the great fear since the epidemic swept through much of Asia early this year -- that the H5N1 virus would mutate, acquire the capacity to pass from person to person and trigger a pandemic in a population without resistance -- was still a long way from reality, Rai said.
"It is still very remote," he said of a case stemming from an 11-year-old girl who died with bird flu-like symptoms in Kamphaeng Phet province in north-central Thailand while her mother was away working.
The mother returned home to see the ailing girl in hospital and died after the cremation of her daughter, who never was confirmed to have had bird flu.
WHO and Health Ministry officials are meeting to review the case, in which the mother's sister is in hospital with similar symptoms, although lab test results have not yet been finalized.
The ministry is also holding a meeting of directors of all state-run hospitals to review bird flu procedures.
"We have been instructed to go on 100 percent alert," Health Ministry spokeswoman Nitaya Mahaphol told Reuters.
"We are recruiting health volunteers in every village in Kamphaeng Phet to keep a close eye on people down with colds or flu," she said.
"There are over 1,000 villages in Kamphaeng Phet. These private volunteers will report any suspicious case to the ministry and their samples would be sent for screening tests."