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"Time and again, I am running into people from USAID, State and the Pentagon who think they are in Kansas [not Afghanistan]," he said. "My auditors tell me things [about spending plans] and I say, 'you have to be making this up, this is Alice in Wonderland'."

The Nato military operation in Afghanistan, which started shortly after the 9/11 attacks in 2001 and which has been spearheaded by the US, will come to a melancholy close at the end of December with the Taliban insurgency still strong, although it does not hold any major cities.

Under current plans, about 10,000 US troops will remain in the country until 2016, although the administration is under growing pressure to extend their presence because of worries about a Taliban resurgence once the US departs.

Since 2001, the government has appropriated $765bn for the war in Afghanistan, the vast bulk for the defence department but also including some spending at the state department.

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The money to finance both the Iraq and Afghan wars was borrowed and, according to Ryan Edwards at City University of New York, the US has already paid interest of $260bn on that war debt. Under FT calculations based on funds appropriated, $125bn of those interest costs have been allocated to the Afghan conflict.

On top of that there are medical costs already incurred for soldiers who have left the military. Linda Bilmes, a Harvard economist who has done extensive research on the war costs, estimates that medical spending on veterans from both Iraq and Afghanistan has so far reached $134bn.

However, she said it was impossible to assess how much of that spending was related only to Afghanistan because a third of soldiers served in both conflicts and because medical issues such as post-traumatic stress disorder were usually the cumulative result of a series of events rather than one incident.

More from the FT:'This is not over yet' Costly missteps mark US road to war Cost of the Afghanistan war

The Afghan conflict has led to other increases in public spending that are significant but difficult to isolate. As well as the separate war funding it has received since 2001, the Pentagon's "base" budget, which covers all its other costs, has also seen a dramatic increase, with the wars in Afghanistan and Iraq one of the main factors.

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Afghan war costs US $1 trillion