At least six endangered one-horned rhinos have been killed since January, including two this month, by poacher gangs at the Kaziranga National Park in Assam, say officials.
"Six rhinos poached in about 100 days, including two killed in the past week, is a matter of grave concern," said park warden Utpal Bora.
The 430 sq km park, 220 km east of Assam's main city Guwahati, is home to the single largest population of the one-horned rhinoceros.
As per latest figures, 1,855 of the world's estimated 2,700 rhinos live in the wilds of Kaziranga -- their numbers ironically making them a favourite target for poaching.
"We are certain that the recent hunting for their horns was done at the behest of a very organised international poaching syndicate that has pumped in lot of funds to attract shooters to kill the animals," Bora said.
Carcasses of the six adult rhinos bereft of their horns were found at the park. The poachers had used rifles and carbines to hunt the animals.
"We have mobilised all the resources and have stepped up security in the park. The villagers have joined us in our fight against poaching," another park ranger said.
Organised poachers kill rhinos for their horns, which many believe contain aphrodisiac qualities besides being used as medicines for curing fever, stomach ailments and other diseases.
Rhino horn is also much fancied by buyers from the Middle East who turn them into handles of ornamental daggers. Elephant ivory tusks are primarily used for making ornaments and decorative items.
Profits from the illegal rhino horn trade are staggering - rhino horn sells for up to Rs 1.5 million ($35,000) per kg in the international market. The fresh incidents of poaching come at a time when authorities believed the endangered one-horned rhinos were charging back from the brink of extinction.
"There was a time when poachers slaughtered about 50 rhinos annually in the early 1990s. But things have slowed down in recent years due to stepped up vigil. Now all off a sudden we see a spurt in poaching again," Bora said.
Five rhinos were poached last year and seven in 2005.
According to government estimates, poachers have killed about 500 of the beasts during the past two decades.