HANOI, Feb 19 (Reuters) - Dead poultry have been found in rivers and streams in northern Vietnam, a sign of a possible new bird flu outbreak during a prolonged cold spell, officials said on Tuesday.

The Agriculture Ministry said in a report that callers to an animal health department hotline reported large numbers of dead birds in five provinces, but was not specific.

The H5N1 strain of bird flu killed three men in northern Vietnam between Jan. 18 and Feb. 14 during a record-long cold spell. The H5N1 virus seems to thrive best in cool temperatures.

"In recent days the Animal Health Department has received many reports about poultry dying in large numbers in provinces," the Agriculture Ministry-run department said in a report on its Web site (www.dah.gov.vn).

"A bird flu outbreak is forecast to emerge in the northern region when cold days extend," it said.

The department, in a separate report, said that bird flu has killed nearly 2,500 ducks and chickens in the northern provinces of Hai Duong, Nam Dinh and Tuyen Quang, bringing to seven the provinces on the government's bird flu watchlist.

Animal health workers have slaughtered the remaining 1,900 birds at the three infected farms.

Doctors confirmed at the weekend that a 7-year-old child from the northern province of Hai Duong had the virus. The child has been under treatment in Hanoi along with several suspected cases.

Bird flu has killed 50 people in Vietnam out of 106 infected cases since late 2003, the Health Ministry said.

Officials said they have not been able to control poultry smuggling from northern neighbour China, which reported its latest human death on Monday, bringing its toll to 18.

H5N1 remains mainly a virus of birds, but experts fear it could mutate into a form easily transmitted from person to person and sweep the world, possibly killing millions.

The virus is known to have killed 227 people globally since late 2003, according to the World Health Organisation, not including the latest death in China and two cases in Indonesia in recent days. (Reporting by Ho Binh Minh; Editing by Grant McCool and David Fogarty)

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