BEIJING, Feb 27 (Reuters) - All three Chinese who died of the H5N1 strain of bird flu this year had contact with sick poultry, the World Health Organisation said on Wednesday, adding there was no evidence of transmission between humans.

A 44-year-old woman in the southern Chinese province of Guangdong on Monday became the third human death from the virus this year in China, following cases in Hunan and Guangxi.

"The three recent cases were not unexpected considering the winter season and the fact that we know the virus is still circulating in the environment," the WHO's China director, Hans Troedsson, said in a statement. Bird flu tends to be more active in the cold.

"We have no indications of any larger number of undetected cases," he said.

With the world's largest poultry population, China is at the centre of the fight against bird flu, which scientists fear could mutate into a form that can pass easily between people, sparking a pandemic.

China has reported four outbreaks of the disease in poultry since December -- the latest announced on Tuesday -- as temperatures across the country have hit their lowest in decades.

The WHO said the human cases were isolated and that all had a history of contact with sick or dead poultry before falling ill.

"At this stage of the investigation there is no evidence of human-to-human transmission. All close contacts are under medical observation and are showing no clinical symptoms so far," Troedsson said.

(Reporting by Lindsay Beck; Editing by Nick Macfie)

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