The brazen killing of a rhinoceros at a wildlife park near Paris by assailants who removed a horn valued at nearly triple the price of gold has put zookeepers on notice that poaching could be spreading beyond the killing fields of Africa and Asia.

Demand for the horns is skyrocketing in Asia, where they are ground into a powder and used for medicinal purposes by some who believe it cures everything from cancer to hangovers.

More than 90 zoos in the United States housing rhinoceros adhere to rigorous security requirements and comply with regular inspections for accreditation, said Dan Ashe, president and chief executive of the Maryland-based Association of Zoos and Aquariums.

But he conceded Friday: "Nobody is insulated from this potential; we have made sure our members are aware of what occurred in Paris and they are quite vigilant. Everybody is sobered."

Jacques-Olivier Barthes, a spokesman for the World Wildlife Fund in France, said zoos in Europe may have to change their security procedures in light of the attack. He said there are 300 rhinoceros in Europe, including 111 in Great Britain.

"If there is a change of criminal strategy and a swing from robbery in museums to crude crime of animals, it will also imply a change of means of protection for the zoos and African reserves which are in Europe," he said.

Officials in France said Tuesday that a 5-year-old white rhinoceros named Vince was shot three times in the head by poachers who broke into the Thoiry Zoo. They used a chain saw to remove the rhino's horn.

"Imagine an animal running around with a big gold horn," said Ashe, who said some estimate the horns could sell for as much as $3,000 an ounce, or between $500,000 and $1 million. "The value is extraordinary, and unfortunately there are criminals out there that will take a risk."

"This is a new potential threat," said Michael Hutchins, former director of conservation and science for AZA and a former executive director of the Wildlife Society. "In order to stop this kind of thing, you have to lower the value of these animal parts and increase the punishment so people think twice about taking the chance. But it's quite complicated."

Tom Stalf, president and CEO of the Columbus Zoo and Aquarium and The Wilds in Ohio, said safety of the animals is his top priority, but he declined to share details. "Doing so would make them less effective," he said in a statement.

The head of Britain's National Wildlife Crime Unit said officials would look to assess and improve security at every location in the United Kingdom that housed rhinos in the wake of the rhino killing in Paris, The Telegraph reported.

Industry standards include 24-hour security guards, and Hutchins says most zoos lock up their animals inside secure buildings overnight.

According to the World Wildlife Fund, the international trade in rhino horn has been banned since 1977. It says the primary market is in Vietnam, where some believe it cures cancer. It is also used in Asian medicine to treat fevers, various blood disorders and, in some cultures, hangovers.

Bas Huijbregts, a WWF African species expert, said this is the first instance he knows of a rhino being killed in a zoo.

"A lot of horns have been stolen from museums in Europe," he said. "A new era is born for zoos across the world. They have to secure them as they would secure grandpa's old watch. It also shows that poaching of African rhinos is not just an African problem. Even in well-protected zoos people are able to kill rhinos."

Huijbregts said that despite the killing in France, his organization's emphasis remains on the poaching of rhinos in the wild.