DHAKA, Jan 1 (Reuters) - Nearly 20,000 chickens were culled after the H5N1 bird flu virus was detected at a government poultry farm in the Bangladesh capital, officials said on Tuesday.

The latest infection was at Mirpur, on the outskirts of Dhaka, said Salahuddin Khan, director of the government's livestock department.

Bird flu was first reported near the capital in March and has since spread mainly to northern districts, forcing authorities to kill more than 300,000 chickens.

About 4 million Bangladeshis are directly or indirectly associated with poultry farming, but so far there have been no cases of human infection, government and health officials say.

But experts fear the bird flu virus might mutate or combine with the highly contagious seasonal influenza virus and spark a deadly pandemic which could kill millions of people.

Bird flu has killed more than 210 people in 12 countries since 2003, the World Health Organisation says. (Reporting by Ruma Paul; editing by Roger Crabb)

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