Closing land or sea borders does not stop migrant smugglers but simply shifts their transport routes, the UN Office on Drugs and Crime (UNODC) says in its first global analysis of people smuggling.
At least 2.5 million migrants were smuggled in 2016, while the people who helped them on their journeys earned between $US5.5 billion and $US7 billion ($A7.3 billion-$A9.2 billion) for their illegal services, the Vienna-based UNODC wrote.
Increased border protection measures are not effective unless they are tied to broader policies, the UN experts said.
"Stricter border control measures often increase the risks for migrants and provide more opportunities to profit for smugglers," they wrote.
Between 2009 and 2015, border closures on the land route from Turkey to western Europe led to increasing demand for the dangerous sea route in the eastern Mediterranean.
Governments should not target the shifting routes but the more stable smuggling hubs where migrants flock before starting their long journeys, UNODC advised.
The UN agency also called on countries to open legal immigration channels and to offer alternative income opportunities to poor people who turn to smuggling, as a way to fight this business.
In addition, law enforcers should address the fact that smugglers work with corrupt officials at various levels.
According to the latest available figures, the biggest routes are across the Mediterranean and from Central to North Africa, with some 380,000 migrants each in 2016.
Between 2014 and 2015, at least 735,000 people migrated from Central to North America.
