John Sopko, special inspector general for Afghan reconstruction, found that project "planning, co-ordination and execution" had been weak.

Security costs were likely to rise in the short-term, he added, as private companies hand over to Afghan forces.

The US plans to withdraw its troops from Afghanistan by the end of 2014.

The quarterly report released on Monday also found that several construction projects were so far behind schedule that they were unlikely to be completed before US troops leave the country.

It said "a significant portion" of the US government's $400m (£254m) investment in large infrastructure projects in fiscal year 2011 alone may have been wasted because of the poor planning.

The report also says the US Army has accepted contract construction so poor it prevented multi-million dollar police bases from being used.

The inspector general noted that one base was unoccupied because it has no viable water supply.

"Other deficiencies included leaking fuel lines, unconnected drain pipes, poorly built guard towers, and improperly installed heating and ventilation systems," the report continues.

The inspector general said there was still time to improve the outcome of the projects in Afghanistan, but warned that time was running out.

The report notes that if Congress approves the Obama administration's latest budget request the US will have allocated almost $100bn on Afghan reconstruction.

The inspector general says the US has never provided so much funding over a similar period of time rebuilding another country.

He points out that reconstruction aid to Germany in the aftermath of World War II amounted to less than $35bn in 2011 dollars.

The report pledges "aggressive oversight of these extraordinary funds" to save taxpayer money and improve the outcomes of the projects.

The report was particularly critical of a handful of projects run by the US Afghanistan Infrastructure Fund (AIF), five of which are running six to 15 months behind schedule.

Execution of all but one of the power-sector projects has been postponed, it says.

The report warns these projects may eventually have adverse effects in their local areas "because they create an expectations-versus-reality gap in the affected population or because they lack citizen support".

It goes on to say the defence department, the state department and USAID, which jointly manage AIF, did not make realistic cost estimates or produce adequate plans to maintain the projects.