KOLKATA, India (Reuters) - Workers sprayed roads andmarkets in Kolkata with disinfectants and culled thousands ofbirds as authorities in eastern India battled to stop anoutbreak of bird flu in poultry reaching the crowded city.

Bird flu has spread to 13 of West Bengal's 19 districts andauthorities in the communist-ruled state said they were cullingsick chickens in a private farm about an hour's drive fromKolkata, one of India's biggest cities.

"We are not taking chances as the farm reported bird deathsand preliminary tests suggest bird flu," Anisur Rahaman, thestate's animal resources minister told Reuters.

Over 2 million birds have already been culled, butauthorities now say they will slaughter thousands of morebirds.

They will also ban rearing backyard poultry in infecteddistricts for at least three months.

India was checking hundreds of villagers and health workersfor possible symptoms of bird flu, officials said.

Experts fear the H5N1 strain could mutate into a formeasily transmitted from person to person, leading to apandemic, but there have been no reported human infections inIndia yet.

The World Health Organization has said it is India's mostserious outbreak of bird flu. Authorities say a major problemis that most poultry in the state of 80 million people areraised in backyards and some farmers have resorted to hidingtheir birds from culling crews.

Farmers raise poultry to earn a little extra money or toensure a reliable source of protein. Many also say compensationof $1 a bird is too low.

Authorities have stepped up efforts to ring-fence theoutbreak to keep it spreading to major urban areas in WestBengal and to stop the virus crossing into other states.

Authorities banned selling chicken in Kolkata's Salt Lakearea, eastern India's IT hub that houses offices of leadingfirms such as Cognizant Technology Solutions Corp and IBM.

They were also disinfecting roads and trucks linkingKolkata to neighboring districts.

"We are trying to minimize the chances of the virus hittingKolkata," Deb Dwaipayan Chattopadhyay, a senior healthofficial, said.

Health workers in Kolkata, capital of the world'slongest-serving democratically elected communist government,were also keeping watch on street markets, following reports ofsick birds being smuggled into the city.

Authorities used loudspeakers and distributed leaflets invillages, asking people not eat infected poultry or eat animalsdying suddenly.

Hundreds of goats, pigs and wild birds have died since theH5N1 virus hit West Bengal's Birbhum district earlier thismonth, the fourth outbreak of bird flu in India since 2006. Butthere has been no confirmation these deaths were due to thevirus.

In neighboring Bangladesh, media reports have said hundredsof crows have also died of bird flu. The virus has spread tonearly half of the country's 64 districts since March last yeardespite mass-culling of poultry after outbreaks are reported.

No human infections have been reported in Bangladesh. Butthe health authority has directed civil surgeons in alldistricts to create an isolation unit for treating suspectedhuman cases.

The Indian government says laboratory tests have confirmedthe 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 neighboringBangladesh, currently reeling under bird flu.

Most countries and all Indian states have banned poultryproducts from West Bengal.

(Additional reporting in Dhaka by Ruma Paul; Editing byAlistair Scrutton and David Fogarty)