“I’m blindsided, as are all of us, by this regulatory change,” said Lark Mason, a New York auctioneer who has specialized in antique ivory for three decades. “We all want to save elephants,” he said, but he questioned how “denying the sale of an 18th-century snuff bottle,” among millions of other decorative antiques, will accomplish that end.

In simple terms, the new regulations ban Americans from importing and, with narrow exceptions, exporting any item that contains even a sliver of ivory. The rules do not ban private ownership, but they outlaw interstate sales of ivory items, unless they meet what sellers describe as impossible criteria.

Officials with the United States Fish and Wildlife Service, which plans to have the new regulations in place in June, said drastic measures are needed to help curb the slaughter of African elephants. The animals now number a scant half-million, and conservationists say as many as 35,000 are dying annually to feed the global black-market in tusks.

“The U.S. market is contributing to the crisis now threatening the African elephant,” the Fish and Wildlife Service director, Daniel M. Ashe, told Congress last month. Wildlife officials say only China has a larger legal market for ivory. As for the black market, over the past 25 years, federal agents say, they have seized six tons of ivory smuggled into in the United States.

Still, Craig Hoover, chief of the Wildlife Trade and Conservation branch at the Fish and Wildlife Service, said officials are reviewing adjustments to the regulations. They sought input Thursday at a meeting in Washington, where the give and take was impassioned.