KOLKATA, India, March 20 (Reuters) - Veterinary staff in safety gear began killing thousands of chickens on Thursday after expanding a culling operation to stop bird flu from spreading in an eastern Indian state.

A fresh outbreak of bird flu was detected in West Bengal state's Malda district where about 1,000 chickens had died in the past few days.

Poultry sales in the eastern state fell by about 70 percent since the avian flu hit in January, officials said, but it has had limited impact elsewhere in the country.

India has not reported any human bird flu cases so far.

Veterinary staff were now killing not only the poultry at the affected farm in Malda but also about 44,000 chickens and ducks in nearby areas to ensure the virus didn't spread.

In January, the H5N1 virus affected 13 of the state's 19 districts, including Malda, but in the following weeks some 3.4 million birds were culled by the authorities who claimed to have contained what the World Health Organisation described as the worst outbreak of bird flu in India.

Anisur Rahaman, the state's animal resources minister, said federal authorities had given the go-ahead to cull chicken after confirming the latest outbreak to be of the H5N1 strain. "Our teams are doing the job," he said.

An outbreak of the virus was reported in neighbouring Murshidabad district earlier this month, the fifth in India since 2006.

Officials say the virus could have originated from neighbouring Bangladesh which is struggling to contain a massive outbreak. (Reporting by Tamajit Pain; Editing by Krittivas Mukherjee)

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