Bird flu has spread to nine of West Bengal's 19 districts and there were reports of more bird deaths in another five districts in the state of 80 million people.

"The virus is most likely to spread to other areas within the infected zones and nearby districts, but we are now trying to stop it from hitting Kolkata," said Surjya Kanta Mishra, WB’s Health Minister.

The H5N1 bird flu virus has been confirmed in two districts so far. Authorities have not confirmed H5N1 in other districts but say it is likely to be the same strain.

The World Health Organization has said it was India's most serious outbreak since the strain was first detected in the country in 2006.

Thailand said on Thursday the strain was confirmed in poultry in a northern province, while Bangladesh has been struggling to contain an outbreak that has affected more than two dozen districts. West Bengal borders Bangladesh.

The H5N1 virus has also resurfaced in Europe, while earlier this week an Indonesian man tested positive for the virus. Nearly 100 people have died of bird flu in Indonesia.

Scientists fear the strain could mutate into a form easily transmitted from person to person, leading to a pandemic that could kill millions around the globe.

Authorities found avian influenza in dead bird samples from Hooghly district, a little more than an hour's drive from the crowded West Bengal capital, Kolkata.

More than five million people live in Kolkata, a city dotted with rows of large houses, crumbling colonial buildings, glittering glass offices and congested slums.

Hundreds of health officials searched markets in the city's narrow lanes, looking for sick birds smuggled in from infected districts. Health officials were also checking birds at the biggest poultry farm in the state near Kolkata.

Strict Surveillance

"A strict surveillance has been put in place and we are checking every single truck coming into the city," said Anisur Rahaman, the state's animal resources minister. "We are making every effort to save the city from the virus."

West Bengal is ruled by the world's longest-serving democratically elected communist government and it is facing huge criticism from the opposition and the central government for a slow response to the outbreak.

Authorities in the neighbouring state of Bihar said on Wednesday they will cull chickens along its border with West Bengal as a precaution.

Authorities on Wednesday intercepted a truck carrying 10,000 chicks in West Bengal's Birbhum district, the epicentre of the latest outbreak of bird flu in India. The truck was apparently trying to leave the state. Officials buried the chicks alive in a ditch, villagers said.

Authorities say is difficult to contain the outbreak in West Bengal because the virus has affected mostly backyard poultry, reared in the thousands by villagers all over the state.