The deadly H5N1 strain of bird flu was found among poultry in Malda, infecting seven of the 19 districts in West Bengal, according to state officials.

"There is every chance of the virus spiralling out of hand if it's too late," said Sanchita Bakshi, the state health services director.

Nepal meanwhile banned the import of poultry products from India. In Bangladesh, authorities culled thousands of chickens after the virus spread to Natore district.

At least 24 million people live in West Bengal's seven affected districts. Officials worry the virus could spread to humans and were collecting random blood samples from villagers.

Experts say the H5N1 strain could mutate into a form easily transmitted from person to person, leading to a pandemic.

"We are taking all precautionary measures and checking human samples with symptoms of cough and cold," said A C Mishra of the National Institute of Virology in Pune.