HANOI (Reuters) - Dead poultry have been found in riversand streams in northern Vietnam, a sign of a possible new birdflu outbreak during a prolonged cold spell, officials said onTuesday.

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

The H5N1 strain of bird flu killed three men in northernVietnam between January 18 and February 14 during a record-longcold spell. The H5N1 virus seems to thrive best in cooltemperatures.

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

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

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

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

Doctors confirmed at the weekend that a 7-year-old childfrom the northern province of Hai Duong had the virus. Thechild has been under treatment in Hanoi along with severalsuspected cases.

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

Officials said they have not been able to control poultrysmuggling from northern neighbor China, which reported itslatest human death on Monday, bringing its toll to 18.

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

The virus is known to have killed 227 people globally sincelate 2003, according to the World Health Organisation, notincluding the latest death in China and two cases in Indonesiain recent days.

(Reporting by Ho Binh Minh; Editing by Grant McCool andDavid Fogarty)