BEIJING (Reuters) - China has announced a bird flu outbreakamong poultry in its unusually cold south, a day after twowomen, one in China and one in neighboring Vietnam, died of thevirus.

The poultry outbreak, first noticed on February 17 in Zunyiin the southwestern province of Guizhou, had killed nearly4,000 birds and triggered the culling of more than 238,000,Xinhua news agency said late on Monday, citing the Ministry ofAgriculture.

China has reported four outbreaks of the disease in poultrysince December, when average temperatures across the countryhit their lowest in decades. Bird flu tends to be more activein the cold.

Experts fear the H5N1 strain could mutate or combine withthe highly contagious seasonal influenza virus and spark apandemic. With the world's biggest poultry population andhundreds of millions of farmers raising birds in theirbackyards, China is seen as crucial in the global fight againstthe disease.

China has reported three confirmed human deaths from birdflu this year, the most recent in the southern province ofGuangdong, neighboring Hong Kong.

The woman, a 44-year-old migrant worker, probablycontracted the H5N1 virus from sick poultry she kept in herbackyard, Hong Kong government health officials said.

The spate of cases is a concern for a country that has theworld's biggest poultry population, many of them backyard birdsroaming free. China has struggled to combat the virus with massinoculations for birds and an education campaign for those whohandle them.

State-run Voice of Vietnam radio said on Tuesday a womanteacher had died from H5N1 in the north of the country, thefourth death from the virus this year.

The 23-year-old woman died on Monday at a Hanoi hospitalafter falling sick as she ate chicken in her home province ofPhu Tho, the radio quoted the Health Ministry as saying.

In communist-run Vietnam, official announcements are oftenmade in state-run media.

The woman's death is the fourth out of five people infectedby bird flu so far this year in Vietnam after an extended coldspell in northern provinces.

Excluding the two deaths on Monday, bird flu has killed 232people among the 366 known cases globally, among them 50 deathsin Vietnam, the World Health Organization has said.

(Writing by Nick Macfie; Editing by Sanjeev Miglani)