MURSHIDABAD, India (AFP) - India's battle against its worst-ever outbreak of bird flu suffered a new blow Thursday as rain forced a halt to culling in West Bengal, an official said.

The virus has already spread to over half the state and the government there has declared the outbreak a crisis, having already admitted it was falling behind in its fight against the deadly H5N1 strain of avian flu.

"Culling has been stopped for the time-being because of the rain -- it will start immediately after the rains cease," state animal resources minister Anisur Rahaman told AFP in Murshidabad, 230 kilometres (140 miles) north of the state capital Kolkata.

Doctors and veterinarians from neighbouring states were arriving in Kolkata to join the culling teams fanning out across the state, the minister said.

But rains, which began early Thursday, had turned many of the rural dirt roads into mud rivers, complicating the battle against the outbreak -- the third and by far the worst to hit India.

Although India has so far not had any human cases of bird flu, Rahaman said he feared the disease would spread to humans with hundreds of people reporting flu symptoms and children "playing with chickens" in affected villages.

Ten out of West Bengal's 19 districts have been affected by bird flu -- representing more than half of the eastern state of 80 million people.

"We were not prepared for a such disaster, we're now getting ready to combat the crisis. We've asked human and veterinary doctors in government hospitals to join the culling as well as private doctors," the minister added.

"We're on a war-footing," he said, speaking from the town near the border with Bangladesh.

It is from Bangladesh that the bird flu is believed to have come from, and authorities there say it has been erupting sporadically since last February.

"We have targeted the culling of at least 2.2 million chickens," Rahaman said. "We have now 650 teams involved in the culling operations. We're trying to get more teams to speed up the culling."

People typically catch bird flu by coming into direct contact with infected poultry. Experts fear a pandemic if the H5N1 strain mutates into a form easily transmissible between humans.