A rodeo cowboy, a Chinese businessman, a Vietnamese nail salon owner and a US antiques expert are among eight people snared so far this year in the largest rhino horn smuggling bust in US history.

US investigators say more arrests are on the way as a new task force sets its sights on a motley crew of organized crime players that experts say is driven by smugglers from Ireland, China, Vietnam, the United States and elsewhere.

Their product is more valuable than cocaine or gold. A single rhino horn can sell for up to half a million US dollars in Vietnam and parts of Southeast Asia where urban legend touts it as a potent aphrodisiac and even a cure for cancer.

"It is a substantial problem," said Edward Grace, deputy chief of law enforcement for the US Fish and Wildlife Service (FWS).

He described his agency's efforts to halt illegal rhino horn trafficking as a "priority."

"The middle men here are buying rhino horn for $5,000 a pound and by the time it makes it to Vietnam it is selling for $25,000 a pound," said Grace, adding that a 20-pound rhino horn could fetch up to $500,000 at its final destination.

To get a share of the cash, would-be profiteers in the United States are scouring for rhino horns, mainly from trophies of the animals that were hunted legally in South Africa and brought back to the United States in recent decades, Grace said.

Meanwhile in Europe, thefts from museums, zoos and auction houses have escalated.

Last year alone, Europol documented 60 thefts -- including 74 rhino horns and eight rhino heads -- from nearly every country in Europe.

"This has turned out to be involving so many different countries that Irish police reached out to us and we got involved," said Soren Kragh Pedersen, chief of media and public relations at Europol.

In July, Europol issued a warning that "significant players within this area of crime have been identified as an Irish and ethnically Irish organized criminal group, who are known to use intimidation and violence to achieve their ends."

Indeed, it was a pair of Irishmen who first drew the attention of US authorities in 2010.

Richard O'Brien and Michael Hegarty of Rathkeale in County Limerick were arrested after paying undercover agents in Colorado about $17,000 for four black rhino horns. They said they would ship them to Ireland with furniture to avoid detection.

They were charged with conspiracy, smuggling and money laundering, and served six months in a US prison.

Grace described the men as belonging to a gang known as the Rathkeale Rovers, "an organized crime element out of Ireland dealing in rhino horns."

They also are known to belong to the Irish travellers community, historically caravan-dwelling nomads with their own distinct culture and traditions.

-- 'Similar to a drug cartel' --

Grace said he believes the illegal rhino horn trade "is really being fueled by the Irish travellers."

After those two arrests, the US Fish and Wildlife Service put together a task force to investigate rhino horn theft cases nationwide.

In February, a sprawling operation that involved 150 agents from the Homeland Security Department and the Internal Revenue Service, as well as FWS undercover buyers of rhino horn, orchestrated a major takedown.

It was prompted by the discovery of $337,000 cash in the luggage of rodeo cowboy Wade Steffen at Long Beach Airport in California.

Grace described Operation Crash, in which "crash" refers to a herd of rhinos, as "the largest seizure and the largest number of arrests at one time in the United States involved in a rhino horn investigation."

Though no Irish were nabbed in the takedown, which involved 37 horns at a value of $8 million to $10 million, more arrests are expected in the coming months, Grace said.

"This case also involves other Irish buying rhino horns in the United States," he told AFP. "I can't go into a lot of details on it."

In addition to Steffen, the suspects have included Vietnamese import-export business owner Jimmy Kha and his girlfriend Mai Nguyen, who owned a nail salon where packages of rhino horns were allegedly being mailed.

Others arrested in Los Angeles include Chinese businessman Jin Zhao Feng who, according to the FWS, allegedly oversaw the shipment of dozens of rhino horns from the United States to China.

Two more arrests on the East Coast involved shadowy deals of rhino horns allegedly being sold outside a gas station off the New Jersey Turnpike and an American antique dealer who sawed rhino horns off a mounted trophy head in a motel parking lot, the FWS said.

"It is similar to an operation of a drug cartel. You have the higher ups who provide the money, the mid-level lieutenants who get the couriers and the smugglers, so you have the whole organized criminal element here," said Grace.

In the United States, it is illegal to sell most kinds of rhino horns across state lines and none may be imported or exported without a special permit.

The maximum penalties are a $250,000 fine and five years in prison for conspiracy and trafficking of endangered species, and $100,000 and one year in prison for violating the Endangered Species Act.

After the February arrests, US attorney Andre Birotti vowed to "continue to target traffickers in the United States who support a heinous industry without any concern for the welfare of this planet's overall environmental health."

Since illegal trafficking fuels poaching of endangered rhinos abroad, "part of the responsibility worldwide to help protect these species falls on the United States," said Grace.