ROME, Jan. 15 (UPI) -- Italian police said they arrested dozens of people involved in an alleged Nigerian drug- and human-trafficking ring.

The operation is accused of forcing hundreds of women into sex slavery, ANSA said Tuesday. Organized crime investigators said 51 Nigerian citizens were arrested in Italy and 15 were arrested in other European countries.

Italy's National Anti-Mafia Prosecutor Piero Grasso said the Nigerian gang was reportedly working closely with Colombian cocaine traffickers. Police said they found a log book in a drug lab outside Amsterdam containing the names and cell phone numbers of 300 couriers.

He said the Nigerian clans represent "'an enormous danger," ANSA reported.

Nearly two dozen Nigerian traffickers and couriers were arrested in an operation last fall in the United States and a number of European countries, the news service said.