A new report by the Democratic majority staff of the Senate Foreign Relations Committee raises serious concerns about the security of hundreds of U.S. diplomats who will remain in Iraq to support the government after the departure of the last 50,000 American troops at year's end.

While Iraq has undergone a "remarkable transformation" since the height of the civil war in 2007, with a more than 90 percent reduction in violence and an elected government now seated, "these advances remain fragile, uneven and reversible," said the report released at a committee hearing on the issue on Tuesday.

Al Qaida's Iraqi affiliate remains a threat, serious sectarian tensions persist and the country has yet to resolve potentially incendiary issues like a new oil law and the status of the city of Kirkuk, it said. All that poses significant security risks to an estimated 17,000 U.S. diplomats, military trainers, support personnel and private security contractors.

"The diplomatic mission that remains will be an initiative of unprecedented size and complexity, currectly projected to consist of some 17,000 individuals on 15 different sites, including 3 air hubs, 3 police training centers, 2 consulates, 2 embassy branch offices and 5 Office of Security Cooperation sites," said the report. "The State Department is scheduled to assume full security responsibilities in a still dangerous and unpredicable environment."

The report said that the State Department and Pentagon bureacracies are having problems coordinating the transition from the military mission to the diplomatic mission, such as agreeing on how many helicopters should remain in Iraq or be sent to Afghanistan.

Finally, the report warned that the success of the State Department-run mission will require a "cohesive and sustainable funding mechanism" at a time when congressional support "has been undermined by a constrained fiscal environment and war fatigue."