Conservationists said Wednesday that they expect Indian authorities soon to ban a cattle drug blamed for killing more than 90 percent of the country's vultures.

Millions of long-billed, slender-billed and oriental white-backed vultures have died in South Asia after eating cattle carcasses tainted with diclofenac, an anti-inflammatory and painkiller given to sick cows.

Vultures play a vital role in disposing of carcasses, keeping down populations of stray dogs and rats that also feed on dead cattle and can spread disease among humans.

But India's government has refused to ban diclofenac until a viable alternative is found, because cattle are crucial to the country's rural economy.

The British journal PLoS Biology reported in January that meloxicam, a drug similar to diclofenac, was effective in treating sick cattle and posed no significant danger to vultures.

The alternative has been proving safe in tests and "a ban should be announced within the next couple of months," Asad Rahmani, the director of the Bombay Natural History Society, said Wednesday.

A final test — giving the drug to buffaloes and then letting vultures eat the animals' meat — is to be conducted next month.

"Since the drug didn't affect vultures when given orally to the birds, it should be safe for use," said Vibhu Prakash, who leads a private vulture breeding program and is a member of the team carrying out the tests.

Prodipto Ghosh, the top bureaucrat in the federal environment ministry, did not confirm that the government planned to ban diclofenac in coming months.

Eliminating diclofenac will not do enough to revive the slow-breeding scavengers, and there must be more breeding programs to restore vulture populations, said Prakash, whose program aims to release 100 pairs of birds within 15 years.

Tens of millions of vultures played a key role in South Asian ecosystems before the introduction of diclofenac in the early 1990s. Now, populations of the three species are thought to have dropped by as much as 97 percent, according to Britain's Royal Society for the Protection of Birds.

The World Conservation Union, which calls itself the world's largest conservation network, has listed the three vulture species as critically endangered, the category applied to animals closest to extinction.

"It's a small dying population," Rahmani said. "One carcass with diclofenac is enough to kill 50 vultures."

Vulture deaths also threaten the customs of India's ancient Zoroastrian community, which uses vultures to dispose of their dead.

Zoroastrians consider the earth and fire too sacred to use for either burial or cremation, and traditionally leave their dead atop towers, to be consumed by vultures.