Two South African men have been charged by the US government with conspiracy to sell illegal rhinoceros hunts to American hunters, money laundering and secretly trafficking in rhino horns.

The charges were part of an 18-count indictment unveiled against Out of Africa Adventurous Safaris, as part of a multi-year investigation by the US Fish and Wildlife Service into wildlife trafficking crimes, known as Operation Crash.

Dawie Groenewald, 46, and his brother, Janneman Groenewald, 44, are also charged with mail fraud and violating the Lacey Act, which makes it a crime to sell products from illegal animal hunts.

Documents filed in federal court describe 11 rhino hunts that took place on the suspects' property in Limpopo Province in South Africa.

The indictment says the brothers travelled throughout the United States to attend hunting conventions and gun shows, where they sold hunts at their game farm in Musina, formerly known as Messina.

"The indictments unsealed today are the first that have been brought under Operation Crash that involved the direct killing of wild rhinos," US Fish and Wildlife Service director Dan Ashe said in Montgomery, Alabama, on Thursday.

"The fact that these defendants allegedly used American hunters to execute this scheme, killing 11 critically endangered African rhinos, is appalling enough, but not as appalling as the brutal tactics that have been employed," he told reporters.

"In one case, a horn was butchered off of a live tranquilised rhinoceros with a chainsaw."

Another rhino "had to be shot and killed after being repeatedly wounded by a bow", the indictment says.

The suspects are in South Africa and US officials are seeking their extradition to the US.

The hunts were illegal because the suspects did not obtain the necessary permits, US officials said.

The American hunters who took part have not been charged.

Demand for rhinoceros horn - which is made from keratin, the same material in hair and nails - has skyrocketed in recent years, largely driven by demand from Asia, where the powdered horn is valued for its supposed medicinal properties.

Experts say the demand for rhino horn has led to the rapid decline of the species by more than 90 per cent since 1970.

All kinds of rhinos are protected under US and international law.

South Africa is in the midst of a growing rhino poaching crisis. At least 700 rhinos have been killed so far this year.

AF, AP