A report by the office of the Special Inspector for Afghanistan Reconstruction found that Washington had set unrealistic expectations for itself after the US-led invasion in 2001 and massively overestimated its ability to build and reform government institutions.

"Despite some heroic efforts to stabilize insecure and contested areas in Afghanistan between 2002 and 2017, the program mostly failed," Special Inspector General John Sopko said as he presented the report in Washington.

"This happened for a number of reasons, including the establishment of a set of unrealistic expectations about what could be achieved in just a few years' time."

The report found that the military pressured aid groups to build schools and infrastructure in areas that were still being contested by the Taliban, leading to the failure of many projects.

Most of the $4.7 billion in stabilization funds was spent after 2009, when a surge in troops to Afghanistan saw large area cleared of the Taliban -- a rush of Western experts then tried to build institutions in these areas.

But the "large sums of stabilization dollars the United States devoted to Afghanistan in search of quick gains often exacerbated conflicts, enabled corruption and bolstered support for insurgents," the report states.

"Opportunities for corruption and elite capture abounded, making many of those projects far more harmful than helpful," it adds.

The SIGAR analysis found that Washington had set expectations and programs not properly tailored for Afghanistan, and noted that successes in stabilizing Afghan districts rarely lasted longer than the physical presence of coalition troops and civilians.

"Under immense pressure to quickly stabilize insecure districts, US government agencies spent far too much money, far too quickly, in a country woefully unprepared to absorb it," SIGAR noted.