The general's report carries a direct call for extra troops
The US mission in Afghanistan will "likely result in failure" unless troops are increased within a year, the top general there has said in a report.
Gen Stanley McChrystal made his assessment in a copy of a confidential report obtained by the Washington Post.
He recently called for a revised military strategy in Afghanistan, suggesting the current one is failing.
More than 30,000 extra US troops have been sent to Afghanistan since May - almost doubling the US contingent.
The number of US troops in Afghanistan is already set to rise to 68,000 by the end of the year.
But in his latest assessment, Gen McChrystal is quoted by the Washington Post newspaper as saying: "Failure to gain the initiative and reverse insurgent momentum in the near-term [next 12 months] - while Afghan security capacity matures - risks an outcome where defeating the insurgency is no longer possible."
He warned that "inadequate resources will likely result in failure".
Gen McChrystal said that failure to provide adequate resources "also risks a longer conflict, greater casualties, higher overall costs, and ultimately, a critical loss of political support".
"Any of these risks, in turn, are likely to result in mission failure."