BEIJING (Reuters) - All three Chinese who died of the H5N1strain of bird flu this year had contact with sick poultry, theWorld Health Organisation said on Wednesday, adding there wasno evidence of transmission between humans.

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

"The three recent cases were not unexpected considering thewinter season and the fact that we know the virus is stillcirculating in the environment," the WHO's China director, HansTroedsson, said in a statement. Bird flu tends to be moreactive in the cold.

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

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

China has reported four outbreaks of the disease in poultrysince December -- the latest announced on Tuesday -- astemperatures across the country have hit their lowest indecades.

The WHO said the human cases were isolated and that all hada history of contact with sick or dead poultry before fallingill.

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

(Reporting by Lindsay Beck; Editing by Nick Macfie)