UNITED NATIONS - The use of mercenaries in armed conflict has increased, particularly this year in Africa where governments used paid foreign soldiers to fight their own people, a United Nations study said Tuesday.

The study by a panel of experts said the growing activities of private security companies were challenging the respect for human rights in countries they were hired. A lack of international regulation of mercenaries has aggravated human rights issues.

The study said the regime of slain Libyan leader Moammar Gadhafi earlier this year hired mercenaries from Eastern Europe and Africa to crack down on pro-democracy protests.

The U.N. also said it had "considerable evidence" that in Ivory Coast earlier this year, the government of President Laurent Gbagbo, who refused to relinquish power after losing elections in November 2010, used 4,500 mercenaries to keep him in power. Gbagbo surrendered in May to the winner of the elections, Alassane Ouattara.

"Mercenaries pose a threat not only to security, but also to human rights and potentially to the right of peoples to self-determination," said Faiza Patel, who chaired the study.

The study said mercenaries were used in the past to fight in wars between countries but that recently they have been used in internal conflict.

It said private military and security companies receive between $20 billion and $100 billion per year.

But in Iraq and Afghanistan, contracts and grants provided by Washington to private security firms exceeded $206 billion in 2011, the study said. Private firms are contracted to provide security and are not necessary involved in conflict.