Preliminary tests on wild storks at a theme park in Ho Chi Minh City showed the birds might be infected with an avian influenza virus, a Vietnamese official said on Friday.

Huynh Huu Loi, director of the city´s Animal Health Department, said more tests were being done to see if they had the H5N1 virus which has killed 42 people in Vietnam since 2003 but has not resurfaced for nearly eight months.

"We have yet to look deep enough into the H5 component of bird flu virus, but the first results found the storks have influenza type A," Loi said.

H5N1 is an influenza type-A virus.

The management of the park had been asked to destroy 53 storks, he added.

Wild birds are natural hosts of bird flu viruses and often don´t show symptoms but can pass the viruses to poultry. H5N1 can kill chickens within 24 hours of infection.

Vietnamese officials say a failure to control waterfowl, which can be silent carriers of bird flu, made the country vulnerable to new outbreaks and wild birds believed to carry H5N1 would soon migrate from the north, raising the risk of infection.