Since October 3, at least 36 people have been killed in floods and landslides across the state, as days of torrential rain destroyed hundreds of homes, washed away tea estates, and left roads impassable.

The deluge also swamped large parts of Gorumara and Jaldapara national parks, protected reserves in the Himalayan foothills that shelter elephants, bison, tigers, and the endangered one-horned rhinoceros.

"The two wildlife sanctuaries... have been submerged under floodwater forcing the animals to stray out," West Bengal forest minister Birbaha Hansda told AFP.

"A herd of 30 elephants strayed out and were seen running through water, trumpeting loudly... We don't know how many elephants will survive," she said.

A leopard was spotted "negotiating choppy water", she said, but has not been seen since.

Other animals could not escape.

"The carcasses of dead animals have started emerging on the riverbanks," she said, listing two leopards, a rhino and several bison and deer among the dead.

Some rescues were successful.

"Two captive elephants were brought to pull out an adult rhino and two elephant calves stuck on a river bank," she added.

India's one-horned Asian rhino population has almost tripled in the past four decades thanks to conservation and anti-poaching efforts, from 1,500 four decades ago to more than 4,000 today.