(Adds official comments)

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.

The government's adviser for livestock, C.S. Karim, told a news conference on Tuesday. "There is no cause for immediate panic ... we are closely monitoring the bird flu situation. We will make it mandatory for all poultry farms to register in order to ensure proper monitoring."

There are around 150,000 poultry farms in Bangladesh, with an annual turnover of $750 million, officials said.

Karim cautioned against getting close to migratory birds now flocking the country's marshes, rivers and lakes.

"We have asked all (villagers and bird lovers) not to be close to the guest birds that fly in every winter, because they may carry the deadly virus," Karim said. (Reporting by Ruma Paul, additional reporting by Masud Karim; Editing by Charles Dick)

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