The report says sexual exploitation is only one aspect of human trafficking
The world must do more to confront the largely unstudied and neglected phenomenon of people-trafficking, the United Nations has said in a report.
So little is known about the problem, says the report, that no estimate can be given of the number affected.
The report also points to a more basic problem: the lack of a common understanding of what human trafficking is, and whom it affects.
But it is "a crime that shames us all", said the UN's Antonio Maria Costa.
In the report - "Global Report on Trafficking in Persons", released by the UN Office on Drugs and Crime (UNODC) - the UN paints a picture of a shadowy form of human slavery little understood by governments, and only rarely adequately tackled.
The report points out that the most commonly used term for the problem - "people-trafficking" - itself emphasises the transaction aspects of the crime, rather than the day-to-day experience of modern enslavement.
And it suggests the trafficking phenomenon is little understood in all its forms from child soldiering to sweatshop labour, domestic servitude, and even entire villages in bondage.
The report cites statistics suggesting that sexual exploitation is the most common form of human trafficking (at 79%, followed by forced labour at 18%).
But it says this itself may be an "optical illusion", because "sexual exploitation is highly visible in cities or along highways while forced labour is hidden".
"We only see the monster's tail," Mr Costa says.
"How many hundreds of thousands of victims are slaving away in sweat shops, fields, mines, factories, or trapped in domestic servitude? Their numbers will surely swell as the economic crisis deepens the pool of potential victims and increases demand for cheap goods and services", he said.
The report also highlights another little-understood aspect of human-trafficking: the fact that female offenders have a more prominent role in people-trafficking than in any other crime, with women accounting for more than 60% of convictions in Eastern Europe and Central Asia.
The report does applaud the "tremendous progress" made by countries in combating a crime it says has only recently been acknowledged - saying that over the space of just five years until 2008, the proportion of member states with legislation outlawing the major forms of trafficking rose from a third to four-fifths.
But it says most countries' conviction rates rarely exceed 1.5 per 100,000 people - "below the level normally recorded for rare crimes... and proportionately much lower than the estimated number of victims".
"It is sick that we should even need to write a report about slavery in the 21st Century," said Mr Costa.
In order to increase the conviction rate, the UN argues, countries and multilateral organisations need to do more to understand the problem itself.
"What we know is the tip of the iceberg, but have no assessment of the iceberg itself," Mr Costa told the BBC.