There's a certain vicious logic employed by traffickers who run human cargoes - many destined to be enslaved as prostitutes - into the United States. Some of the time you can hide your victims in plain sight.

Thousands of women are ferried across the US-Mexican border into one of the world's most lucrative slave markets. But unlike their counterparts in the 18th-century slave trade, none wear chains.

Indeed, some believe they will soon embrace the American Dream.

To US officials, such hopefuls are often indistinguishable from the hundreds, maybe thousands, of people who try to cross the border every day.

But instead of legitimate work, the traffickers' victims are prostituted as sex slaves in a dark and savage American nightmare.

Unlike the open slavery practised in the past, the victims of modern bondage live in a shadowy world where threats of violence guarantee their silence and protect their oppressors from legal retribution.

"If you get caught with guns and drugs you'll get a long prison term," says Rick Castro, a deputy sheriff with the San Diego County Sheriff's Department and a veteran of the war against modern slavers.

"But if you're a trafficker you've already told your victims that if they talk to the cops they'll be killed or raped. Or their family members back home will be killed. So there's less chance of being caught."

Little wonder that many victims keep quiet. Only 100 or so human trafficking cases have made it into US courts.

Yet trafficking is at epidemic proportions ranking behind drugs and weapons smuggling among the world's most profitable crimes.

The actual number of US slaves (forced to work without pay as farm workers, domestic servants and prostitutes) is unknown. The State Department estimates that between 14,500 and 17,500 people are trafficked into the US each year, far lower than some calculations.

"Maybe half that annual number, or more, become sex slaves," says Kevin Bales, president of Free the Slaves, a Washington-based non-governmental organisation. "It's a very hidden crime. And it's one in which the crime is not an event, like a mugging reported to the police, but a process."

Bales estimates that 27 million people are enslaved worldwide.

In France some 90 per cent of female prostitutes are believed to be trafficking victims. Bales estimates that up to 250,000 slaves exist in the US. "The basic rule of trafficking is that people move from poorer to richer countries," he says.

Many victims come from Russia, Eastern Europe, Asia, and Latin America. But the trade is pervasive. Women are traded from, say, Mali to Ghana, and from Ghana to Nigeria, then from Nigeria to Italy.

While some victims are smuggled across borders, others travel on legal visas or as mail-order brides. Lured by promises of a better life, some even buy their own tickets. Many naively believe they will pay off the cost of travel expenses in a few months.