In what animal conservationists hailed as a “significant milestone” in the global fight against elephant poaching, the Obama administration on Tuesday announced a ban on nearly all ivory sales in the United States.

Part of the new National Strategy on Wildlife Trafficking , the ban is intended to end a trade that threatens to wipe out the world’s largest land animal. The administration said that for the first time, vendors must prove beyond any doubt that ivory offered for sale complies with the Endangered Species Act.

Administration officials said authenticity can be established only with a permit — from the U.S. government or a foreign government — showing that the ivory was imported before a ban in 1989. Antique ivory, older than a century, is also exempted, with proof of age. Even with a permit, the sale of ivory will not be allowed across state boundaries, according to an administration official.

Conservationists said those restrictions are especially important to combat illegal poaching and trafficking in ivory, which have exploded in recent years.

Driving the robust black market are the value of ivory, estimated at $1,500 a pound by the time it reaches China, and of rhinoceros horn, worth more than its weight in gold, according to authorities, with estimates as high as $25,000 a pound.

The United States is the world’s second-largest market, behind China, for illegal wildlife artifacts. The legal sale of ivory in the United States and around the world helps to disguise black-market sales, U.S. prosecutors and other law enforcement officials say.

Officials at the U.S. Fish and Wildlife Service said U.S. leadership is key, as was shown last year when the nation crushed several tons of seized ivory, leading China and France to follow suit.

Elephants that once roamed the African plains by the millions now number fewer than a half-million, and about 35,000 are killed each year. Last year, more than 1,000 rhinos were poached in South Africa alone.

“Every piece of ivory comes from a dead elephant,” said Jeff Flocken, regional director for North America at the International Fund for Animal Welfare. “The U.S., by coming up with this national strategy, is saying they take wildlife crime very serious. We’re looking at our own house and how we can address our own consumption.”

It has been impossible for law enforcement, through observation, to identify whether ivory was purchased before 1989 or was antique. Shifting the burden to the seller to prove an artifact’s age is a huge development, Flocken said.

The World Wildlife Fund’s president and chief executive, Carter Roberts, called the proposed changes “a significant milestone in the global fight against wildlife crime . . . reflecting the fact that it has grown into one of the most profitable criminal industries in the world, estimated at $10 billion annually.”

The administration made combating illegal trafficking in ivory and rhino horn a priority for several federal agencies, including the departments of State, Justice, Homeland Security and Interior, which oversees the Fish and Wildlife Service.

The strategy has three major parts: strengthening enforcement; reducing demand for black market wildlife artifacts in the United States and the world; and building strong partnerships with world governments and nonprofit groups. Tuesday’s announcement focused solely on tougher regulation of ivory sales, complementing a ban since the late 1970s on the sale of rhinoceros horn in the United States.

Commercial exports of ivory products also will be banned, except for proven antiques and items covered by specific exceptions under the Endangered Species Act, officials said. A special Fish and Wildlife rule that relaxed restrictions on trading ivory will be revoked under the strategy.

Permits for importing elephant sport-hunting trophies collected by American hunters in African nations will be reduced to two a year.

In addition to elephants and rhinoceros, animals ranging from endangered tigers to turtles are slaughtered to sell hides and other parts, often for the benefit of organized criminal syndicates.

President Obama signed an executive order creating a federal task force and a separate advisory commission on wildlife trafficking in July. Tuesday’s announcement was the product of the task force’s first recommendations.

The announcement followed recent congressional approval of nearly $50 million to fight illegal wildlife trafficking this year. It was made ahead of Thursday’s London Conference on the Illegal Wildlife Trade.

The conference is described on a British government Web site as a meeting of global leaders to “eradicate the illegal wildlife trade and better protect the world’s most iconic species from the threat of extinction.”