Workers sprayed roads and markets in Kolkata with disinfectants and culled thousands of birds as authorities in West Bengal battled to stop an outbreak of bird flu in poultry reaching the crowded city.
Bird flu has spread to 13 of West Bengal's 19 districts and authorities in the communist-ruled state said they were culling sick chickens in a private farm about an hour's drive from Kolkata, one of India's biggest cities.
"We are not taking chances as the farm reported bird deaths and preliminary tests suggest bird flu," Anisur Rahaman, the state's animal resources minister told Reuters.
Over 2 million birds have already been culled, but authorities now say they will slaughter thousands of more birds.
They will also ban rearing backyard poultry in infected districts for at least three months.
India was checking hundreds of villagers and health workers for possible symptoms of bird flu, officials said.
Experts fear the H5N1 strain could mutate into a form easily transmitted from person to person, leading to a pandemic, but there have been no reported human infections in India yet.
The World Health Organisation has said it is India's most serious outbreak of bird flu. Authorities say a major problem is that most poultry in the state of 80 million people are raised in backyards and some farmers have resorted to hiding their birds from culling crews
Farmers raise poultry to earn a little extra money or to ensure a reliable source of protein. Many also say compensation of $1 a bird is too low.
Authorities have stepped up efforts to ring-fence the outbreak to keep it spreading to major urban areas in West Bengal and to stop the virus crossing into other states.
Authorities banned selling chicken in Kolkata's Salt Lake area, eastern India's IT hub that houses offices of leading firms such as Cognizant Technology Solutions Corp and IBM.
They were also disinfecting roads and trucks linking Kolkata to neighbouring districts.
"We are trying to minimise the chances of the virus hitting Kolkata," Deb Dwaipayan Chattopadhyay, a senior health official, said.
Health workers in Kolkata, capital of the world's longest-serving democratically elected communist government, were also keeping watch on street markets, following reports of sick birds being smuggled into the city.
Authorities used loudspeakers and distributed leaflets in villages, asking people not eat infected poultry or eat animals dying suddenly.
Hundreds of goats, pigs and wild birds have died since the H5N1 virus hit West Bengal's Birbhum district earlier this month, the fourth outbreak of bird flu in India since 2006. But there has been no confirmation these deaths were due to the virus.
In neighbouring Bangladesh, media reports have said hundreds of crows have also died of bird flu. The virus has spread to nearly half of the country's 64 districts since March last year despite mass-culling of poultry after outbreaks are reported.
No human infections have been reported in Bangladesh. But the health authority has directed civil surgeons in all districts to create an isolation unit for treating suspected human cases.
The Indian government says laboratory tests have confirmed the H5N1 strain in at least two of West Bengal's 19 districts, but reports from 11 other districts were likely to be the same.
Authorities said the virus could have come from neighbouring Bangladesh, currently reeling under bird flu.
Most countries and all Indian states have banned poultry products from West Bengal.