They say that the spread of the H5N1 virus means that even more chicken and duck will have to be killed than was originally estimated.

On Monday officials said that around 2m birds would need to be culled - a figure that will now rise.

Health experts have warned that the outbreak could get out of control.

The H5N1 strain of bird flu is regarded as highly pathogenic and can also cause disease and death in humans.

However, most human victims have contracted the disease through close contact with affected birds.

There is little evidence that the virus can be transmitted easily between humans.

'Very serious'

West Bengal's Minister for animal resources and development, Anisur Rehman, told the BBC there are currently 650 culling teams in the state who have killed about 430,000 birds with another 200,000 expected to be destroyed on Wednesday.

Villagers are reported to be reluctant to hand over birds

They have now been ordered to extend their chicken and duck killing operation from seven to nine districts.

He said that one of the districts most recently affected, Hoogly, is close to Calcutta and contains the state's largest chicken hatchery - which has not yet been affected.

Mr Rehman said that another district recently hit by the virus, Cochbihar, is close to the border with Bangladesh.

West Bengal has sealed a stretch of its border with Bangladesh, which has been fighting to contain the spread of bird flu since March last year.

Experts in Bangladesh have warned that the outbreak of the virus is far worse than the government is reporting.

"Bird flu is now everywhere. Every day we have reports of birds dying in farms," leading Bangladeshi poultry expert MM Khan told the AFP news agency.

"Things are now very serious and public health is [in] danger," he said, alleging that farmers were reluctant to report new cases.

Logistical challenge

The BBC's Amitabha Bhattasali in Calcutta says the authorities in West Bengal face the same problems, with reports of people hiding chicken and duck to stop them from being killed.

The pace of culling has been too slow, critics say

Our correspondent say the authorities face a major logistical challenge, because many of the chickens in infected areas roam around in the open and in many cases have to be chased and caught before they can be killed.

On Monday a team of officials from Delhi told the state government that the pace of the culling operation is too slow.

At the same time, the state government warned that the virus was getting "out of hand".

It has appealed to eight other Indian states - including Maharashtra and Tamil Nadu - for assistance in the culling operation.

Correspondents say that the problem is made worse because many poor and illiterate farmers are sometimes misinformed about basic hygiene.

Dead birds are reported to have been dumped in village wells and ponds by people not aware of the risks from the H5N1 virus.