The explosive leak uncovering America's vast surveillance program highlights the risks Washington takes by entrusting so much of its defense and spy work to private firms, experts said Monday.

From analyzing intelligence to training new spies, jobs that were once performed by government employees are now carried out by paid contractors, in a dramatic shift that began in the 1990s amid budget pressures.

Edward Snowden, the 29-year-old man whose leak uncovered how spy agencies sift through phone records and Internet traffic, is among a legion of private contractors who make up nearly 30 percent of the workforce in intelligence agencies.

After the attacks of September 11, 2001, the use of contractors boomed, as government agencies turned to private firms in the global hunt for terror suspects, touting it as a cost-effective way to avoid a permanent increase in the number of civil servants.

As a "contractor alley" rose in the suburbs of northern Virginia outside Washington, the increasing reliance on contractors by the Pentagon and spy services has often been criticized as wasteful and possibly corrupt.

But some former intelligence officers and experts warn that it also opens up the spy agencies to big security risks.

The contractors who wear a "green badge" to enter government offices may lack the ethos and discretion of career intelligence officers who wear the "blue badge," according to John Schindler, a former analyst at the National Security Agency and counterintelligence officer.

In a series of tweets, Schindler, who now teaches at the Naval War College, heaped scorn on Snowden for spilling secrets.

But he said it was not surprising the disclosure came from a "green badge" holder and suggested sensitive information technology jobs should not be contracted out.

"Been telling my CI (counter intelligence) peeps for years that NSA & IC (intelligence community) only 1 disgruntled, maladjusted IT dork away from disaster (esp IT contractor)...oh well," he wrote.

Systems administrators are the 21st century equivalent of the Cold War-era "code clerks," he said, as they may not hold a high rank but have access to vital information.

Most contractors are former military or intelligence officers, and America's top spy chief, James Clapper, once worked at Booz Allen Hamilton, the same firm that employed Snowden. Another former national intelligence director, Michael McConnell, also worked at the firm before and after holding the director's post.

Booz Allen has profited heavily from intelligence work, reportedly earning $1.3 billion or 23 percent of its total revenue from contracts with spy agencies.

Former CIA director and defense secretary Robert Gates has voiced concern that too much sensitive work has been farmed out to private companies.

"You want somebody who's really in it for a career because they're passionate about it and because they care about the country and not just because of the money," he told the Washington Post in 2010.

A special website lists job openings for those with security credentials, clearancejobs.com, with positions advertised such as "Intelligence Analyst 3/Targeter" for Northrop Grumman.

"The primary function of a Specialized Skills Officer is to collaborate with a team of intelligence professionals in support of HUMINT operations against priority targets," said the notice for a workplace in McLean, Virgina.

But the threat of damaging leaks may have less to do with a dependence on contractors and more to do with a younger generation's distrust of Washington, said James Lewis, a former senior official and cyber security expert at the Center for Strategic and International Studies.

Private contracting does not in and itself pose a serious threat to keeping secrets, Lewis told AFP.

"It's a risk because of the differing attitudes of generations," he said.

"People who haven't been in the federal service for a long time, who have this view of government shaped by the popular culture are probably more inclined to do this."

He noted that the most extensive leak of US classified documents came not from a contractor but a low-ranking soldier in the US Army, Private Bradley Manning, who is on trial on espionage charges after admitting to handing over hundreds of thousands of secret files to the WikiLeaks website.