Seven rhinos have been killed in an attack on a South African farm, bringing the total number killed this year to 570 as black-market demand for their horns continues to surge, the government said Tuesday.

At least seven rhino were brutally killed at a game farm near the northwestern town of Rustenburg over the weekend, according to the Department of Environmental Affairs.

Three people have been arrested in connection with the latest killings, which included the rhinos' being dehorned and badly mutilated, according to the SAPA news agency.

The rhinos are victims of a booming demand for their horns, which are believed to have medicinal properties in some traditional Asian medicine.

The number of rhinos poached in South Africa has risen from 13 in 2007 to 333 in 2010 and 448 last year.