A Thai health official warned on Monday that authorities must prepare for an expected second onslaught of bird flu, and Vietnam reported the region's 19th fatality from the virus that has ravaged poultry farms across Asia.

The World Health Organization (search), meanwhile, said China may already have human cases of bird flu, although the country hasn't reported any. It said while the government has been sharing information about known outbreaks, it might not be aware of everything happening in the country.

"WHO feels it is conceivable that there may be human cases, given the extent of the outbreaks in poultry," said Dr. Henk Bekedam (search), the agency's Beijing representative.

The comments come after a weekend report on state television said China has "not yet discovered any cases of humans catching the disease."

Malaysia, which has so far kept free of the disease and restricted poultry imports from affected countries, also cleared a 40-year-old pet-store owner and bird breeder of fears he caught the virus. He was quarantined on Saturday when he displayed flu symptoms.

Ten Asian governments — Cambodia, China, Indonesia, Japan, Laos, Pakistan, South Korea, Taiwan, Thailand and Vietnam — have battled bird flu in recent weeks and officials in the United States confirmed an outbreak in the state of Delaware on Friday.

But Pakistan, Taiwan and the United States are reporting milder versions of bird flu than the H5N1 strain (search) that has jumped to humans in Vietnam, where it has killed 14 people, and Thailand, where it has killed five and is suspected of sickening 23 others, nine of whom have died. Tens of millions of chickens have been slaughtered across the region.

Charan Trinwuthipong, director general of Thailand's Department of Communicable Disease Control, said the "first wave of bird flu outbreak has passed," in an apparent reference to the country's almost complete cull of poultry in bird flu-affected areas.

He said the Agriculture Ministry is trying to eliminate the sources spreading the disease, "but we don't know when the second wave will come, and we don't trust the situation. So the Public Health Ministry is being as careful as possible."

A 27-year-old Vietnamese man, from southern Binh Phuoc province, died Monday at Ho Chi Minh City's Hospital of Tropical Diseases, officials said. His family had kept chickens.

Vietnam has 19 confirmed cases of people becoming infected with bird flu — 14 of them have died, three remain in hospitals and two have recovered.

Also Monday, Philippine Health Secretary Manuel Dayrit said the government has banned poultry imports from the U.S. state of Delaware after officials there confirmed an outbreak of a different strain of bird flu.

The Philippines also burned 353 imported lovebirds from the Netherlands that passed through Thailand on their way to the Philippines because of fears they could have been infected by the virus, officials said.