VIENNA, July 16 (Reuters) - Turnover of cross-border organised crime is about $870 billion a year, more than six times the total of official development aid, and stopping this "threat to peace" is one of the greatest global challenges, a U.N. agency said on Monday.

The most lucrative businesses for criminals are drug trafficking and counterfeiting, the U.N. Office on Drugs and Crime (UNODC) said, launching an awareness-raising campaign about the size and cost of cross-border criminal networks.

"Millions of victims are affected each year as a result of the activities of organised crime groups with human trafficking victims alone numbering 2.4 million at any one time," UNODC said in a statement.

The total estimated figure of $870 billion is equivalent to 1.5 percent of the world's gross domestic product, it said, warning that crime groups can destabilise entire regions.

"Stopping this transnational threat represents one of the international community's greatest global challenges," UNODC Executive Director Yury Fedotov said.

A spokesman said it was the first time the agency had compiled an estimate for transnational organised crime, using internal UNODC and external sources, so there were no comparative figures to show any trend.

The report drew on International Labour Organisation (ILO) data on the cost of human trafficking as well as information about counterfeit goods compiled by the Organisation for Economic Cooperation and Development (OECD). (Reporting by Fredrik Dahl; Editing by Stephen Nisbet)