Threatened shark species are being used to make shark fin soup, a delicacy in Chinese cuisine, in several US cities, according to an unprecedented study based on DNA testing.

Thirty-three different species of sharks turned up in samples collected in 14 cities and analyzed at Stony Brook University's Institute for Ocean Conservation Science in New York.

"US consumers of shark fin soup cannot be certain of what's in their soup," said Demian Chapman, who co-led the DNA testing, in a statement Wednesday. "They could be eating a species that is in serious trouble."

Scalloped hammerhead sharks, listed as endangered by the International Union for Conservation of Nature, was among the species found on the menus of US restaurants where shark fin soup can sell for as much as $100 per bowl.

Others included smooth hammerheads, school sharks and spiny dogfish, all listed as vulnerable to extinction, as well as a variety of near-threatened species such as bull and copper sharks.

"This is further proof that shark fin soup here in the United States, not just in Asia, is contributing to the global decline in sharks," said Liz Karan, of the Pew Environment Group, a foundation that supported the study.

"Sharks must be protected from overfishing," said Karan, manager of Pew's global shark conservation program, "and any international trade in these vulnerable and endangered species must be tightly regulated."

The study marks the first time that DNA testing has been used to ascertain the different kinds of sharks used to make shark fin soup in the United States on a large nationwide scale.

Samples were collected in Albuquerque, New Mexico; Atlanta, Georgia; Boston; Chicago; Denver, Colorado; Fort Lauderdale, Florida; Houston; Las Vegas; Los Angeles; New York; Orlando, Florida; San Francisco; Seattle and Washington.

The study, to which the Field Museum in Chicago contributed its DNA expertise, is to figure prominently in a television program on sharks on the Discovery science channel next week.

Up to 73 million sharks are killed around the world every year to supply a Asian-driven demand for shark fin soup, the Singapore branch of the WWF conservation organization says.

Since sharks are slow-growing and mature at a late age, they are particularly vulnerable to the danger of extinction, with potentially serious knock-on effects for lower rungs of the oceanic food chain.