A U.S. and British-backed code of conduct has been signed in Geneva by major private security operators, including some operating in Iraq and Afghanistan.
The voluntary code is aimed at preventing abuse and reining in excess violence in lawless conflict zones.
Almost 60 firms, including U.S. firms Triple Canopy, Xe Services (formerly Blackwater), and Britain's G4 Security signed up, while the code has the backing of 35 countries.
Blackwater became notorious in 2007 when its guards opened fire in a Baghdad square, killing 17 civilians.
The code includes a pledge that staff cannot invoke contractual obligations or "superior orders" in a conflict zone to justify crimes, killings, torture, kidnappings, detentions, or summary executions.
Companies also pledge that their staff will not use firearms except in self-defense.
Afghanistan's government has ordered private security firms to disband and leave the country amid growing anger among ordinary Afghans.
compiled from agency reports