Arnold Fields, the government inspector who oversees American contracts in the country, said hundreds of new Afghan outposts, barracks and garrisons are "seriously behind schedule, making it doubtful that the construction efforts would keep pace with recruitment and training." Of 884 projects for completion over the next two years, only 133 are finished. Another 78 are under way, but 673 have not begun.
Poor planning, weak management and corruption were behind the delays, Mr Fields told a Congressional commission investigating US wartime spending.
Building Afghanistan's security forces almost from scratch is the cornerstone of allied plans for a gradual withdrawal from the country.
NATO has vowed to build the police and army to a total of 305,600 by October. By the end of 2014 Afghans are expected to have taken over fighting duties. However, the alliance has said it will only pull out if Afghan forces are ready.
NATO says it is meeting recruitment and training goals, but Mr Fields, a retired US Marine general, said it was well behind on construction targets.
Weak planning could also mean the £7.4 billion budget would deliver buildings "inadequate in order to meet the long-term or short-term needs of the security forces".
Congress has repeatedly questioned whether the billions annually pumped into the nine-year-long Afghan campaign are being stolen or wasted while the US economy struggles out of an economic downturn.
Mr Fields announced his resignation earlier this month following criticism from senators that his office was incompetent and unable to weed out corruption and waste in US contracts.
Commanders in charge of logistics and construction in Afghanistan concede some projects are behind, but deny they are affecting plans to hand over to Afghan forces.
Col John Ferrari, US deputy commander for programs, said: "On the ultimate metric of fielding the Afghan security forces, we are ahead of schedule."