Last year alone, poachers killed at least 20 rhinoceroses in and around Assam's sprawling Kaziranga national park. This year, the toll has already reached seven, officials said.

The 430-sq km park, with around 1,800 rhinoceroses, has the world's largest population of this primitive mammal. Two other reserves in Assam, Pobitora Wildlife Sanctuary and Orang National Park, have around 150 rhinoceroses.

Three rhinoceroses were killed in Orang this year.

Countrywide, the rhino population could be just over 2,200, including in West Bengal, Bihar and Uttar Pradesh, said Prabal Sarkar of the NGO Wildlife Trust of India (WTI).

At the turn of the sanctuary, the rhino roamed almost all over the Gangetic plain. Its numbers fell sharply over the decades because of depleted grassland habitat and frequent hunting -- to feed the illegal rhino horn trade.

Experts say these animals are killed for its horn, which is used as aphrodisiac by some tribes in the northeast and in some Asian countries.

"A rhino horn could fetch between Rs.500,000 (about $11,000) and Rs.1 million in the international market. Traders also sell the horn in pieces or in powder form," said Sarkar, who studies the rhino horn trade in the region.

"Its demand in the Asian traditional medicine market is perhaps the single largest factor for its poaching throughout its home range," he said.

A small population of around 400 rhinoceroses can also be found in Nepal's Chitwan National Park, Bardia National Park and Suklapantha National Park.

In the 1980s, poachers commonly used the pit-poaching technique to trap rhinoceroses in Kaziranga. The method involves digging a pit and covering it with leaves and grass. The animal gets trapped when it steps on the camouflage. Poachers then kill the animal.

This involves constant monitoring of the animal, mainly to establish its path and to dig a trap, which is not easy. Hence, poachers at times electrocute the animal.

Last year, forest guards found a tranquiliser inside the park, raising fears the instrument might have been illegally brought to kill rhinoceroses. Such guns don't make any noise and their detection is near impossible.

Gunshots alert forest guards. Hence poachers carry out pit-poaching or electrocution. But sometimes they also use noiseless sophisticated guns.

Last year, poachers shot dead a pregnant rhinoceros even as authorities were looking for clues for previous crimes of rhino poaching at the Kaziranga park.

In another horrific act this year, a rhinoceros and her calf were repeatedly shot on the periphery of Kaziranga.

The mother was still alive, and her calf lay dead beside her, while poachers made way with its horn.

The incidents led to widespread public protests and prompted Assam Forest Minister Rokybul Hussain to call for a Central Bureau of Investigation (CBI) inquiry.

The CBI is yet to receive a formal request from the Assam government.

Hussain had vowed to up the maximum jail term for crimes under the Wildlife (Protection) Act 1972 from seven years to 10 years.

But Kaziranga park director S N Buragohain does not believe this will help in a major way. He said the existing provisions of the act were adequate to deal with anti-rhino crimes. Instead, more efforts should be made to collect crucial evidence from the crime scene to help to crack poaching cases in courts.

But for this, "we need staff who could solely deal with previous cases. Most of our staff are managing the everyday work and cannot be spared for such work", Buragohain said over telephone from Bokakhat, about 200 km from Guwahati.

"We need more people, mainly from the law background, who could plan and execute the cases in court."