"The new administration has emphasized the importance of a responsible drawdown of US forces but has not yet defined this term," said a report by the Government Accountability Office (GAO), the investigative arm of Congress.

US President Barack Obama said last month that most of the 140,000 US combat troops would be withdrawn by the end of August 2010.

Meanwhile, up to 50,000 would remain until the end of 2011, when an agreement between Iraq and the US requires all troops to leave.

"In further defining this strategy and revising the Joint Campaign Plan for Iraq, the administration will need to clarify what conditions need to be met to undertake this drawdown responsibly," according to the report.

The administration should be "prioritizing the conditions that would allow US troops to draw down" and "should also consider how the United States would respond if it does not achieve the conditions necessary for a responsible drawdown within the security agreement timetable" agreed by the United States and Iraq.

The GAO said the logistics of withdrawing tens of thousands of troops within two years and "the removal of equipment and material will be a massive and expensive effort."

The US government also needs to outline diplomatic and civilian efforts that will replace the American military mission, which undertook many traditionally civilian tasks in Iraq, according to the GAO.