WASHINGTON  - The main US foreign aid agency is preparing to switch from private security contractors in Afghanistan to Afghan government-provided security this month under a new policy mandated by Afghan President Hamid Karzai, raising concern in Washington that this could put US civilians at greater risk.

US Agency for International Development Administrator Rajiv Shah says the agency may be able to negotiate waivers from the policy for some major infrastructure projects so that they could continue to have access to private security.

But US AID officials also said this week that only 25 percent of US-funded development projects in Afghanistan require security guards, suggesting the changeover to Afghan government-provided security this month that Karzai has ordered may not be so dramatic.

“Seventy-five percent of our assistance portfolio does not require private security contractors today. So a lot of our partners, and a lot of the way we are doing business is not affected by this at all,” Alex Thier, Shah’s assistant for Afghanistan and Pakistan programs, said in an interview.

Private security contractors working for foreign companies, who have numbered in the thousands, are no longer allowed on aid and development programs after March 20 under Karzai’s decree. If these programs want armed escorts or guards for their compounds, they are supposed to contract with a branch of the Afghan police, the Afghan Public Protection Force (APPF).

Karzai has long been critical of private contractors and other “parallel institutions” in Afghanistan and wants them under the control of the Afghan government.

Yet it’s far from clear that the Afghan Public Protection force can provide the same level of security.

Security has deteriorated markedly across Afghanistan over the past two years. A spate of bloody protests and attacks on US soldiers in Afghanistan following the recent burning of copies of the Koran by US troops have raised more concern on Capitol Hill about the safety of American soldiers as well as civilians in programs to improve Afghan health, education and the economy.

“In fact, this new violence calls into question whether the current assistance strategy can be successful if Americans are not even safe to work in ministries alongside Afghan staff who should be our partners,” Representative Kay Granger, chairwoman of the House subcommittee on foreign aid, said this week.