KOLKATA, India (AFP) - The World Health Organisation Thursday warned that an outbreak of bird flu in eastern India was far more serious than two previous outbreaks.

"More serious risk factors are associated with this current outbreak than previously encountered, including that the affected areas are more widespread and because of proximity to extended border areas," the organisation said.

Health officials are engaged in culling 400,000 birds in several districts of India's heavily populated eastern West Bengal state bordering Bangladesh, which is also struggling with the virus.

The slaughter started a day after India's agriculture ministry confirmed that the death of 35,000 birds in West Bengal was due to the deadly H5N1 strain.

According to India's federal health ministry, 35,525 poultry in West Bengal's Birbhum district as well as 288 birds in a state-run poultry farm in the state's Dinajpur district had died, but it added the outbreak appeared to be localised.

But villagers in Margram, at the centre of the outbreak, told an AFP photographer accompanying culling teams that they had sold sick and dead birds as meat for 10 rupees (25 cents) a kilo (2.2 pounds).

Officials in New Delhi said advisories had been sent to states neighbouring West Bengal in a bid to contain any possible spread. Flights originating from Kolkata had taken chicken off their menus, reports said.

An isolation centre has been opened in a hospital near the affected area and 300 health workers have been sent with medicines and protective gear, Anisur Rahaman, West Bengal animal resources minister said.

The outbreak is the third in India, home to 1.1 billion people, since 2006.

Humans are typically infected by coming into direct contact with infected poultry, but experts fear the deadly virus may mutate into a form easily transmissible between humans.

Wild migratory birds have been blamed for the global spread of the disease, which has killed more than 200 people worldwide since 2003.