"I would say that it is coming under control," David Nabarro told Reuters, "And the final test of whether it's under control would be whether or not authorities are able to really dampen down the new outbreaks of H5N1 particularly in West Bengal."

Bird flu has spread to 13 of 19 districts in West Bengal in the fourth outbreak of the H5N1 strain since 2006, with samples of dead chickens testing positive in two new districts, officials said on Monday.

In neighbouring Bangladesh, the disease has spread to 29 of its 64 districts since March last year despite mass-culling of poultry.

"What the authorities need to try to avoid is the continuous transmission of the H5N1 virus so you almost stop noticing the outbreaks," Nabarro said, sitting in his suite in a five-star hotel in the Indonesian capital Jakarta.

The public health expert is on a four-day visit in the Southeast Asian nation, where he meets ministers and experts to review Indonesia's efforts to stamp out disease.

Nabarro said a major problem is that most poultry in West Bengal and Bangladesh are raised in backyards and farmers have been reluctant to cull their birds in the fear of losing a vital source of income and nutrition.