A record 1,004 rhinos were illegally killed in South Africa in 2013 as demand for the animal's horn skyrockets in Asia, officials say. Most rhino poaching in South Africa, home to both black and white rhinos, has occurred at Kruger National Park, CNN reported. Poachers employed by organized crime groups use helicopters, night vision equipment and guns with silencers in rhino killing expeditions, while South African authorities try to combat poaching with unmanned drones, sniffer dogs and increased security, the news report said. The horns are sold to buyers in China and Vietnam, where they are used for medicinal purposes. Experts say a kilogram of rhino horn is worth about $20,000 -- a single horn weighs about 10 kilograms, or 22 pounds. Conservation groups say there are about 25,000 African rhinos worldwide, most of which are in South Africa. Meanwhile, an American man who purchased a $350,000 permit to kill a black rhino in Namibia has received death threats, CNN said. Hunter Corey Knowlton bought the permit at an auction hosted by the Dallas Safari Club. Although Knowlton said the money he spent on the permit, one of five issued by the Namibian government this year, would go to conservation, he said he had to hire a security team following the death threats.