Eleven people were arrested in Zaragoza, Spain, in connection with the trafficking plot, Europol said
, adding that their criminal network operated mainly in Spain, Italy, Germany and Denmark.
The women reflect a recent wave of Nigerians who have headed to Europe from Benin City, where poverty and traditional witchcraft suffocate opportunity
. The journey can be fraught for women and girls, who face a particular risk of exploitation, experts say.
Promised a better life in Spain, the 16 women were moved by land from Nigeria to Libya, then to Italy by sea, and from there into Spain, Europol said.
"Once on the Spanish territory they were handed over to a madam and forced into prostitution until they paid off their debt," the agency said.
Voodoo threats leveled by traffickers
The women also were pressured under voodoo threats to pledge to pay for their transport and not to report their exploiters to police, Europol said.
"This method, used by criminal organizations with women from Western Africa, aims to control women under threat of death for them or their family members if they do not comply with that commitment," the agency said.
In some cases, traffickers can control their victims using phone contact alone, Europol said.
Women and girls at risk for exploitation
The number of potential sex-trafficking victims arriving in Italy by sea has increased 600% since 2014, with the majority of victims arriving from Nigeria, a July report
by the International Organization for Migration estimates.
Eighty percent of all women and girls arriving from Nigeria to Europe are potential victims of trafficking for sexual exploitation, the group estimates, noting that exploitation of younger girls has been on the rise.