Vietnam, which has been free of human bird flu cases this year, has found the H5N1 virus on a small duck farm, animal health officials said on Monday.

The discovery was made in the southern province of Ben Tre as neighboring countries Laos, China and Thailand have all reported new outbreaks of the deadly H5N1 virus in recent weeks.

"We have received laboratory confirmation that the ducks had the H5N1 virus," an animal health official from Thach Phu district, Ben Tre province, said by telephone.

He said all 84 ducks on the farm were immediately culled on Saturday and that ducks in 14 nearby farms were also tested for the virus, but the results were all negative. No one in nearby villages was sick from the infected birds, the official said.

The source of the virus was unknown but the official said, "what we know so far is that the ducks were raised in a closed farm and had not been let out in the open for quite sometime."

A H5 subtype resurfaced in Vietnam in the past month, mainly in ducks and wild storks, but the Southeast Asian country has had no human infections of H5N1 since December.

Public health officials have told countries such as Vietnam that although the virus remains essentially an animal disease, it can return at any time and could spark a pandemic if it mutates into a form that can pass easily among people.