Reports in the United States claim the CIA hired private contractors from the controversial firm Blackwater as part of a secret campaign to find and kill top Al Qaeda leaders.

The New York Times quotes current and former administration officials who say the CIA hired Blackwater to locate and assassinate top terrorist operatives.

Millions of dollars were spent, but no Al Qaeda leaders were captured or killed.

Former Bush administration officials refused to talk about the specific case from 2004, but defended the use of "surrogates", saying they wanted to hire the best people for the job.

Former CIA director Michael Hayden, who took over in 2006, was reluctant to comment on the reports.

"We still hyperventilate in the agency when people use the word assassinations - assassinations would be in violation of executive order," he said.

"No one is talking about assassinations."

While refusing to discuss the specific case, he defended the use of contractors.

"We routinely use surrogates," he said. "Surrogates are routinely briefed to Congress.

"There are no surprises to Congress that the fact that much of the activity of the agency is done on our behalf by surrogates.

"Keep in mind that surrogates come in a variety of flavours and that an intelligence service like the CIA has the ability to choose among those flavours for different missions."

Michael Chertoff, the former secretary of the Homeland Security Department, which also uses contractors, says it comes down to budget.

"If you don't want to fund the people then you can't complain when you're going to use contractors," he said.

"I think [as] General Hayden put it, if you give a submission but you don't fund slots, then that mission is going to be performed using contractors."

In June, the current CIA director Leon Panetta learned about the plan, pulled the plug, and briefed Congress on the contracts, concerned that they had been kept secret for seven years.

There are now investigations as to why the program was kept secret from Congress.

General Hayden has disputed charges from some of President Barack Obama's Democratic allies that former vice-president Dick Cheney abused his power by ordering the information be withheld.

"The vice-president never told me not to tell Congress about this. The vice-president never told me not to tell Congress anything," General Hayden said.

The US State Department cut ties with Blackwater following allegations of abuse in Iraq, and the firm's operations in Iraq officially ended in May.

The North Carolina-based company renamed itself Xe after the Iraq government banned it in January over the killings of as many as 17 civilians in Baghdad's Nisur Square on September 16, 2007.

Iraqis and critics have repeatedly accused Blackwater of having a cowboy mentality and a shoot-first-ask-questions-later approach when carrying out security duties.