South African rangers on Thursday announced a plan to implant GPS devices in the horns of rhinos in a new effort to combat rampant poaching.

The GPS chips link up to a computer monitoring station where park rangers track the rhinos.

"The animal's movements are then tracked 24/7 and if they are attacked, game rangers will be alerted via the alarms," park enforcement officer Rusty Hustler told the Sapa news agency.

The alarm signal activates if the rhino lies inert for longer than is deemed normal, or becomes unusually active.

Five rhinos in North West province have already been fitted with the device, but more will follow if the programme succeeds.

"If we prove it completely then my consideration will be that all the North West parks that have rhino should have the GPS device," he said.

Poachers stalk rhinos by helicopter and tranquilise them with darts from hunting rifles. The horns are removed while the giant animals lay unconscious.

Rhino poaching has spiked dramatically since 2008 with 227 slaughtered so far this year, almost double the number slain in 2009.

Black-market demand for rhino horn is particularly high in China and Vietnam, where poachers sell the horns for medicinal and ornamental purposes for up to 20,000 dollars per kilo.