Federal prosecutors in New York have charged four African men with trafficking millions of dollars' worth of rhinoceros horns and elephant ivory.

The U.S. Attorney's Office in Manhattan said Thursday the conspiracy involved the poaching of some three dozen endangered rhinoceroses and more than 100 elephants.

Prosecutors say the ring smuggled nearly 420 pounds (190 kilograms) of rhinoceros horns and at least 10 tons of elephant ivory from east Africa to buyers in the United States and southeast Asia.

They say the men from Liberia, Guinea and Kenya hid the contraband in pieces of art like masks and statues.

The case highlights the thriving black market for rhinoceros horn, worth more than gold in some countries because of beliefs in its healing power.

Prosecutors say the black rhinoceros population has fallen to about 5,500 worldwide.