Cuba, France and Montenegro have in the last year reported cases of mercenaries being convicted in their courts, the report said.
The United States alone spends $138 billion a year on private security, much of it in Iraq and Afghanistan in recent years, the report said.
The Pentagon spent $44 billion in 2012 in Iraq and Afghanistan for private military and security forces, it said. The United States paid $3 billion in 2011 just for a five-year security contract on the U.S. Embassy building in Baghdad, the report said.
As the U.S. military presence winds down, much of the private security work is shifting to police work and base support, but the protection of oil company facilities is a new growth area, it said.
Private security on shipping off Somalia's coast is another new market identified by the report, with over 140 companies providing armed guards in the region.
The United Nations itself is another major employer for private security, and the report of the Working Group on the Use of Mercenaries cautioned "there is a risk that, without proper standards and oversight, the outsourcing of security functions by the United Nations to private companies could have a negative effect on the image and effectiveness of the United Nations in the field."