Johannesburg - South Africa's rhinoceros population is under threat from poachers, who are killing the leathery quadripeds in growing numbers for their valuable horns, according to a report released Wednesday.

The Witness daily in the eastern province of KwaZulu-Natal reported that 12 white rhinos had been killed in game reserves in the province so far this year. Their horns were harvested and the carcasses left to rot.

A spokesman for the provincial wildlife authority, Ezemvelo KZN Wildlife, confirmed the killings, which marks a steep increase on previous years.

Another KZN Wildlife official told the paper the loss was a huge compared to previous years. The number of rhinos poached from the province's reserves usually does not exceed two or three a year.

The most recent reported rhino killings in the area took place at Hluhluwe-Imfolozi Park in September, one of South Africa's most famous game parks, north-east of the port city of Durban.

Rangers patrolling the park found the decomposing carcasses of four rhinos, including two adult females, that had been shot with high-calibre rifles.

"The horns of all four animals had been cleanly removed with a sharp instrument, indicating that someone with considerable experience had been at work," Jeff Gaisford, KZN Wildlife spokesman, told the Witness.

The police organized crime unit is investigating the killings, which are believed to be linked to a flourishing black-market trade in rhino horns.

The horn is highly sought after in Asia, particularly China, where it is considered to have medicinal and aphrodisiacal qualities.

South Africa's biggest game reserve, Kruger National Park, last year also reported a slow and steady increase in rhino killings - at least 70 over six years.

Zimbabwe, where an economic crisis has distracted the government from wildlife management, is another prime target for rhino poachers. The country has lost thousands of rhinos - one of the Big Five game animals alongside the lion, leopard, buffalo and elephant - to poaching since the 1980s.