"Although considerable effort and sacrifice have resulted in some progress, many indicators suggest the overall effort is deteriorating," Gen. Stanley McChrystal wrote in a five-page Commander's Summary. Details of McChrystal's assessment were first reported late Sunday by the Washington Post.
"Resources will not win this war, but under-resourcing could lose it," McChrystal also wrote in the 66-page report, which was sent to Defense Secretary Robert Gates on Aug. 30 and is now under review by President Barack Obama.
More than 30,000 extra U.S. troops have been sent to Afghanistan since May, almost doubling the U.S. contingent.
The number of U.S. troops in Afghanistan is already set to rise to 68,000 by the end of the year. There are also some 40,000 troops from other nations, mainly NATO allies.
McChrystal, who took over as military commander in May, is expected to make a separate request for tens of thousands of extra forces to be deployed. He also says that training for Afghan forces needs to be speeded up.
McChrystal also pointed out in the report an "urgent need" to revise strategy. The United States needs to interact better with the Afghan people, McChrystal said, and better organize its efforts with NATO allies.
"We run the risk of strategic defeat by pursuing tactical wins that cause civilian casualties or unnecessary collateral damage. The insurgents cannot defeat us militarily; but we can defeat ourselves," he wrote.
President Obama is also evaluating whether the renewed focus on hunting al-Qaida that he announced just months ago has become blurred and whether more forces will do any good.
"Are we doing the right thing?" he asked during one of a series of interviews broadcast Sunday. "Are we pursuing the right strategy?"
Senate Republican leader Mitch McConnell told CNN on Sunday his party would be supportive of a U.S. troop increase for Afghanistan if it was needed as part of a new strategy, but he said he was troubled by the delay in the decision-making.
"We think the time for decision is now," McConnell said.
Geoff Morrell, a deputy assistant secretary of defense for communications issues, said in a statement the assessment "is a classified, pre-decisional document, intended to provide President Obama and his national security team with the basis for a very important discussion about where we are now in Afghanistan and how best to get to where we want to be."
In the report, McChrystal also provided new details about the sophisticated nature of the Taliban insurgency; criticized NATO forces for focusing more on tackling insurgents than protecting Afghan civilians; censured the Afghan government for lack of action on widespread corruption; and warned that Afghanistan's prisons have become a sanctuary for active insurgents.
All of these factors, McChrystal claimed, have led to a "crisis of confidence among Afghans" in the face of a resilient insurgency.
A senior adviser to Afghan President Hamid Karzai told the BBC that his government was not against more international troops being sent, but their success would depend on where they were sent.
"Our official stance is that until our security forces are strengthened, both in terms of numbers and quality, there won't be a long-lasting peace in Afghanistan," Sebghatullah Sanjar said.
---- Compiled from wire reports and other media sources