WITH their laser-targeting assault rifles, wrap-around sunglasses and black uniforms, the men guarding Afghan president Mohammed Karzai look like US special forces.

But in fact they are American mercenaries, working for private contractors. The special forces who used to guard Karzai flew home last autumn after the Bush administration decided to contract out the job of protecting the Afghan leader.

Their presence is just one manifestation of a dramatic trend towards the privatization of war by the Pentagon.

The use of private contractors is not new. What is unprecedented is that the Pentagon has pushed privatization further than ever, even to the extent of using private contractors for core military functions.

While the US army has shrunk by a third since the end of the Cold War, the private military industry has grown rapidly. It now comprises 1,000 companies generating between $100bn and $200bn (£60bn and £120bn) a year, according to Peter Singer, in a new book, Corporate Warriors: The Rise of the Privatized Military Industry.

Singer said: "This is a huge and growing industry that is increasingly active in a number of conflict zones. The state is usually thought of as having a monopoly on the use of force. But it doesn’t anymore."

During the 1991 Gulf War, there was one private contractor for every 50 soldiers. In Iraq the war involved one private contractor for every 10. The companies were involved in military operations, such as maintaining the weapons on American Stealth warplanes, reconnaissance aircraft and navy ships.

Richard Boucher, the State Department spokesman, said the department awarded the contract to protect Karzai to a private firm called DynCorp - which employs former Special Operations military personnel and CIA officers - because its own security agents did not have the proper training or weaponry to deal with combat conditions in Afghanistan.

But the decision has been criticized by two Congressmen, Henry Hyde, a Republican, who is chairman of the House International Relations Committee, and Tom Lantos, the most senior Democrat.

"Experience with such contractors elsewhere leads us to believe that the presence of commercial vendors acting in this capacity would send a different message to the Afghan people and to President Karzai’s adversaries: that we are not serious enough about our commitment to Afghanistan to dispatch US personnel," they wrote in a letter to the State Department and the Pentagon.

In the Iraq war, private military contractors came to public attention when men in uniform were seen to enter the US buffer zone and cut holes in the border fence to pave the way for the assault on Iraq. Now, in the aftermath of the war they are there in far greater numbers, performing many duties including guarding military installations and blowing up mines, usually in a plethora of unrecognizable uniforms.

At Camp Arifjan, the giant US army base south-west of Kuwait City, the armed figures guarding the gates are civilian employees of Combat Support Associates that provides the army with security, logistics, "live-fire training" and maintenance.

Meanwhile, when a plane carrying US missionaries was shot down by a Peruvian military aircraft in Peru, it emerged the attack was an accidental one, carried out after the missionaries’ plane was mistakenly identified by a US surveillance aircraft flown by private contractors hired by America for its war against drugs.

"We are hiring a private army," said Janice Schakowsky, a Democratic congresswoman who has sponsored two failed bills to try to curb the role of commercial contractors in military roles. "We are engaging in a secret war, and the American people need to be told why."

Critics say it is unclear whether civilian contractors who are captured can claim prisoner of war status or whether they risk being shot as mercenaries. They also point out that contractors are not subject to the military chain of command or to military discipline. While soldiers can face charges for desertion, contractors can simply leave.

During the Iraq war, a US reserve air force colonel in Kuwait was heard to complain that when essential communications equipment maintained by the manufacturer went wrong in the middle of the night, he could not get it fixed because the contractor was not there. "We’re fighting a war and the contractor doesn’t come in until 9am," he said.

But it is argued that civilian contractors provide better value for money, and that they allow the military to concentrate on military duties.

"You don’t need a marine to drive a forklift," said Doug Brooks, head of the industry’s trade group, the International Peace Operations Association.