A permit to hunt an endangered African black rhino sold for $350,000 at a Dallas auction held to raise money for conservation efforts but criticised by wildlife advocates.

Steve Wagner, a spokesman for the Dallas Safari Club, which sponsored the closed-door event Saturday night, confirmed the sale of the permit for a hunt in the African nation of Namibia. He declined to name the buyer.

The Safari Club's executive director, Ben Carter, has defended the auction, saying all money raised will go toward protecting the species. He also said the rhino that the winner will be allowed to hunt is old, male and nonbreeding and that the animal was likely to be targeted for removal anyway because it was becoming aggressive and threatening other wildlife.

But the auction drew howls from critics, including wildlife and animal rights groups, and the FBI said it was investigating death threats against members of the club.

Officials from the Humane Society and the International Fund for Animal Welfare have said that while culling can be appropriate in abundant animal populations, all black rhinos should be protected given their endangered status.

An estimated 4,000 black rhinos remain in the wild, down from 70,000 in the 1960s. Nearly 1,800 are in Namibia, according to the Safari Club.

Critics have said any hunting of a rhino sends a bad message to the public. "This auction is telling the world that an American will pay anything to kill their species," Jeffrey Flocken, North American regional director of the Massachusetts-based IFAW, said. "This is, in fact, making a spectacle of killing an endangered species."

The auction took place in downtown Dallas under tight security. Organisers hoped to at least break the previous high bid for one of the permits in Namibia, which was $223,000, and had said the amount could be as high as $1m. The nation offers five permits each year, and the one auctioned on Saturday was the first to be made available for purchase outside Namibia.

The winning bidder could have come from anywhere in the world and at least some bidders were expected to enter by phone.

About 40 protesters gathered early Saturday evening outside the convention centre where the auction and a pre-auction dinner were to take place. They held signs and chanted.

"There's less than 5,000 black rhinos left on the planet," said Jim Ries, who travelled with his family from Atlanta to protest, "and if our kids ever want to see a rhino left in the wild we can't be pulling the trigger on every one we say is too old to breed."

Poachers long have targeted all species of rhino, primarily for its horn, which is valuable on the international black market. Made of the protein keratin, the chief component in fingernails and hooves, the horn has been used in carvings and for medicinal purposes, mostly in Asia. The near-extinction of the species also has been attributed to habitat loss.