By Subir Bhaumik
BBC News, Calcutta
Thousands of birds have been slaughtered in the state of Assam
Authorities in the eastern Indian state of West Bengal say they have identified the deadly H5N1 strain of bird flu in thousands of dead chickens.
Tests on poultry from two villages around Englishbazar town in the Maldah district have returned positive results, a health official said.
He "few thousand" birds in the area will be slaughtered, he said.
The H5N1 strain of bird flu is regarded as highly pathogenic and can also cause disease and death in humans.
Bird flu has returned to West Bengal after an outbreak in 14 districts of the state in January this year.
Separately, authorities in the north-eastern state of Assam say they have finished culling of more than 300,000 chickens and ducks to control the spread of bird flu in the state.
Poultry in seven districts in the state have turned up cases of bird flu.
Assam Health Minister Himanta Biswa Sarma told the BBC that nearly 100 people had reported fever and respiratory tract infections in the districts affected by bird flu.
"Medical experts are closely examining these people. We have to be doubly sure they have not been infected by bird flu," he said.
Federal medical teams have brought in supplies of equipment as a preventive measure in case the virus spreads to humans.
Bird flu was first detected in Assam in a poultry farm outside the state's capital Guwahati late in November.
Since then it has spread to seven districts - from Goalpara in the western border with Bangladesh to Dibrugarh in the far north of the state.
Assam, West Bengal, Tripura and Manipur states have suffered outbreaks of bird flu during the past year.
Millions of chickens and ducks have been slaughtered after the outbreaks.
While no case of humans being infected by bird flu has been reported from anywhere in India, experts fear the H5N1 virus might mutate or combine with the highly contagious seasonal influenza virus and spark a pandemic that could kill millions of people.
Since the virus resurfaced in Asia in 2003, says the World Health Organisation, it has killed more than 200 people in a dozen of countries.