"You can add to that, obviously, relations between Sunnis and Shi'ites, Arabs and Kurds, Iraq and all its neighbors - none of these questions have been answered," added Harling. "So withdrawing at a high pace, within the context of a framework which gives the U.S. very little flexibility, when all these questions remain unanswered is obviously a gamble."

A recent increase in violence has raised fears that the U.S. drawdown, along with Iraq's political vacuum, could further embolden insurgents.

U.S. officials said last week that they will increase the number of private security forces in Iraq by as many as 7,000. The duties of the temporary contract workers would include protecting U.S. officials.