Two South African men were charged Thursday 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 indictments unsealed today are the first that have been brought under Operation Crash that involved the direct killing of wild rhinos," said US Fish and Wildlife Service Director Dan Ashe.

"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 tranquilized rhinoceros with a chainsaw."

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

The suspects are currently in South Africa and their extradition to the United States is being sought by officials.

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.

- Rhino demand -

The US Justice Department alleges that the Groenewald brothers recruited American hunters at conventions and gun shows, and sold them hunting trips at their ranch in Mussina, South Africa.

Hunters paid between $3,500 and $15,000 for the opportunity.

The South African men allegedly told US hunters that a particular rhino had to be killed because it was a problem to the herd.

They were told that while the carcass and horns could not be exported, the hunters could kill the animal and pose for a picture with it, at a reduced price, the indictment said.

"These defendants tricked, lied and defrauded American citizens in order to profit from these illegal rhinoceros hunts," said US Attorney George Beck of Alabama.

"Not only did they break South African laws, but they laundered their ill-gotten gains through our banks here in Alabama."

Janneman Groenewald lived in the southern US state from 2005 to 2010.

The two men are also charged with selling the rhino horn on the black market, but officials declined to say how much they earned or to whom the sales were made.

No rhino horn has been seized in the investigation.

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 over 90 percent since 1970.

The horn sells on the black market for as much as $35,000 per pound.

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

South Africa is in the midst of a growing rhino poaching crisis, and has seen at least 700 rhinos killed so far this year.