KOLKATA (AFP) - Health officials in India's West Bengal state said on Monday that they were culling chickens on a "war footing" as bird flu spread to new areas of the state at the weekend.

The disease has now been found in 13 of the 19 districts in the eastern state, prompting fears that it may hit the densely-populated capital city of Kolkata.

And the state government said it could revise the target number of birds to be culled up to three million.

"We are worried that the H5N1 virus was confirmed in samples from villages just 22 kilometres (13 miles) from Kolkata," West Bengal animal resources development minister Anisur Rahaman said. "If it is required, culling teams will work throughout the night."

More than 1.5 million birds have already been culled since the outbreak was reported about a fortnight ago.

Authorities raided backyard poultries at night to avoid resistance from locals who, fearing financial losses, would try to hide the birds or lock up their houses to prevent the chickens from being culled.

"There was no resistance, no hostility. They swiftly culled the chickens after paying compensation," Rahaman said.

Poultry owners say they have been devastated by the cull, with the government paying only about 40 rupees (one dollar) for each dead chicken, compared with the 80 rupees they could earn on the market, excluding egg sales.

But the state finance minister Asim Dasgupta has put the losses to the state's poultry industry at only about 10.5 million rupees.

Humans typically catch the disease by coming into direct contact with infected poultry, but experts fear the H5N1 strain may mutate into a form easily transmissible between people. No human cases have been reported in India.