A growing number of private security and military companies are operating 'domestically and internationally' without effective oversight or accountability, the United Nations working group on the use of mercenaries warned on Tuesday.

Presenting its report to the United Nations Human Rights Council in Geneva, the UN working group said that private security companies in such conflict-wracked countries as Iraq, Colombia and Afghanistan are recruiting former policemen and members of the military from developing countries as 'security guards' in their operations.

Once there, those guards in fact become 'militarily armed private soldiers', which is essentially a new way to describe mercenaries, who are often responsible for serious human rights abuses, it said.

War-torn states, the group said, frequently lack the capacity to control and regulate the private companies with national legislation granting immunity to them which are sometimes transnational in some cases.

When this happens, the private guards are only accountable to their employers, and the working group said immunity can soon turn into impunity.

The group, established in 2005 and composed of five independent experts serving in personal capacities, called for the wider ratification of the International Convention against the recruitment, use, financing and training of mercenaries.