A report by the UN's International Labor Organization (ILO) says forced labor generates at least $150 billion a year worldwide in illicit profits.
The new report, released on May 20, says that amount is three times what was originally thought.
Beate Andrees, head of the ILO's Special Action Program to Combat Forced Labor, said at a press conference in Geneva that "two-thirds [of the total profit] is generated through sexual exploitation" of women and girls -- about $99 billion a year.
She said the rest came from forced economic labor, such as agricultural work, domestic work, construction, and mining.
ILO surveys on the forced labor of migrant workers in Moldova showed that more than 45 percent of returned migrants were forced to work abroad for little or no pay. Almost 25 percent said they received late payments.
In Armenia and Georgia, 20 percent of returned migrants also reported working for little or nothing.
Andrees said that annual profits per victim are highest in developed economies.
She said that "this includes EU member states and this can be explained by the value added to the labor in those countries."
However, more than half of all forced laborers work in Asia, plus another 18 percent in Africa, and almost 10 percent in Latin America.
Andrees called on countries to tackle the causes that drive their citizens into the hands of traffickers and gangs that force them to work for little or no money.
She said countries must help protect their poor from abusive lending or indenturing and invest more in education and training to increase job opportunities for vulnerable workers.
She also called on states to promote the rights of migrants and support the organization of workers in industries vulnerable to forced labor.
According to ILO estimates, some 21 million people worldwide are victims of forced labor, trafficking, and modern-day slavery.
The UN calls human trafficking the third-largest international criminal industry after drugs and arms trafficking.