The United States has destroyed six tons of confiscated elephant ivory, in a move wildlife groups hailed as a bold message to criminal traffickers that the ivory is worthless.

The ivory stockpile was pulverized by an industrial rock crusher at a US Fish and Wildlife Service refuge near Denver, Colorado on Thursday.

The move was "a clear message that the nation will not tolerate wildlife crime that threatens to wipe out the African elephant and a host of other species around the globe," the FWS said in a statement.

Witnesses to the event included representatives of African nations and top conservationists from around the globe.

"Rising demand for ivory is fueling a renewed and horrific slaughter of elephants in Africa, threatening remaining populations across the continent," said Secretary of the Interior Sally Jewell.

"We encourage other nations to join us in destroying confiscated ivory stockpiles and taking other actions to combat wildlife crime."

The Philippines, Kenya and Gabon have already destroyed their stockpiles of ivory tusks and trinkets.

The US stockpile was accumulated over the course of 25 years, and was seized during undercover investigations of organized smuggling operations or confiscated at the border.

The FWS said most of the ivory, prior to being seized, was headed for illegal sale in the United States or overseas.

"By crushing its contraband ivory tusks and trinkets, the US government sends a signal that it will not tolerate the senseless killing of elephants," said Carter Roberts, president of World Wildlife Fund.

The multimillion dollar illegal ivory trade has driven the loss of three-quarters of all African forest elephants in the past decade, according to the Wildlife Conservation Society.

"To end the elephant poaching crisis, we know that we must take a three-pronged approach: stop the killing, stop the trafficking, stop the demand," said WCS president Cristian Samper.

The US stockpile destruction "demonstrates its own commitment to eliminating the market for illegal ivory," he said, adding that the United States could do more, such as establish a moratorium on all ivory sales within its borders.

Experts say poachers killed 35,000 elephants last year, at a rate of 96 animals per day.