The State Department is preparing to spend close to $3 billion to hire a security force to protect diplomats in Iraq after the U.S. pulls its last troops out of the country by year's end.

In testimony Monday before the Commission on Wartime Contracting, Patrick Kennedy, undersecretary of state for management, said the department plans to hire a 5,100-strong force to protect diplomatic personnel, guard embassy buildings and operate a fleet of aircraft and armored vehicles.

Underscoring the security risks in Iraq, five American troops were killed Monday in an attack in Baghdad, the largest single loss of life for the U.S. military there since April 2009.

Fewer than 50,000 U.S. troops remain in Iraq. Under a 2008 U.S.-Iraqi security agreement, all U.S. troops are supposed to leave the country by the end of the year, leaving behind only a small military office to oversee arms sales.