WHO said on Thursday it was concerned the H5N1 strain of bird flu was spreading across Asia and there were "mounting opportunities" for the virus to mutate
"We see this as an issue of growing concern that more countries have H5N1 infections among poultry stock," Bod Dietz, the UN health agency's spokesman in Vietnam, said.
"Although we have seen no evidence of human-to-human transmission, the next step would be for that to occur.
"It is impossible to predict a time or date for this but there are mounting opportunities for the virus to alter its form and begin affecting the human population."
Thailand diagnoses virus
The alert came as a Thai official said the country's first case of bird flu had been confirmed in a child. Thailand had been testing three people for the fatal disease.
"The case in Suphan Buri (province) has tested positive, that is for sure," said Nirun Phitakwatchara, chairman of the parliament's Social Development and Human Security committee.
On Wednesday, Thailand's health minister, Sudarat Keyuraphun, urged the public not to panic if cases of bird flu were confirmed.
She said her ministry "has prepared measures to control the spread of the virus". However, she did not elaborate.
Meanwhile, Deputy Agriculture Minister Newin Chidchob said up to six million chickens had been destroyed in the central provinces of Nakorn Sawan and Chachersao provinces.
"The case in Suphan Buri (province) has tested positive, that is for sure,"
Thai Social Development and Human Security committee
Officials said many were infected with bird cholera and
respiratory diseases other than bird flu.
Thailand, a major poultry exporter, has about 1.1 billion chickens at local farms.
Some farmers did not destroy their chickens at first, but the ministry ordered them to cull the birds to prevent a massive blow to exports of Thai poultry, Newin said.
"If bird flu is found in Thailand, we will be banned from exporting all chicken for a period of six months," he said.
Newin said one reason for killing the birds was that authorities fear farmers would feed them banned antibiotics to keep them disease-free, and that use of the drugs could threaten Thailand's lucrative chicken exports.
The European Union, a major customer, bans poultry with any traces of certain antibiotics believed to be dangerous for human consumption.