Washington (CNN) -- The United States' nation-building projects in Afghanistan can claim only limited success and the troubled nation risks sinking into deeper crisis after a U.S. troop withdrawal, according to a congressional report issued Wednesday.
The two-year investigation found that, despite the roughly $18.8 billion spent in aid to Afghanistan over the past decade, more than to any other country, the United States should "have no illusions" about the value of its programs aimed at stabilization.
The Senate Foreign Relations Committee report details the use of aid dollars in areas of Afghanistan cleared of the Taliban and urges the administration to reconsider how the money is being spent.
The Senate panel's report said the Obama administration is pursuing an aid strategy in line with counterinsurgency theories that "deserve careful, ongoing scrutiny to see if they yield intended results."
"Foreign aid, when misspent, can fuel corruption, distort labor and goods markets, undermine the host government's ability to exert control over resources, and contribute to insecurity," it said.
The sobering document comes as debate rages over the extent of U.S. involvement in Afghanistan and President Barack Obama considers the size of an initial U.S. troop drawdown and how best to hand over responsibilities now under military control.
Obama discussed transition with Afghan President Hamid Karzai in an hourlong video teleconference Wednesday, White House spokesman Jay Carney said. Obama has not made any decisions yet regarding a drawdown, Carney said.
"We welcome the report, even though we do not endorse all of the conclusions of it," he said. "It's important to note that Afghanistan has made significant progress, and the presumption that our assistance has contributed little and that Afghanistan has made no progress is just simply wrong and we disagree with that."
He cited the training of Afghan forces as an example.
Also Wednesday, the Senate committee grilled Ryan Crocker, tapped by Obama to be the next U.S. ambassador in Kabul, at confirmation hearings. Crocker, the former ambassador to Iraq, acknowledged that problems like corruption stand as impediments to success in Afghanistan.
"We wrestled with the same thing in Iraq and you don't get positive change overnight," he told the senators.
"I am under no illusions of the difficulty of the challenge," Crocker said. "If Iraq was hard -- and it was hard -- Afghanistan in many respects is harder."
About 97% of Afghanistan's gross domestic product is related to international military and donor community spending, the report said, citing the World Bank.
"Afghanistan could suffer a severe economic depression when foreign troops leave in 2014 unless the proper planning begins now," the report said.
"The administration is understandably anxious for immediate results to demonstrate to Afghans and Americans alike that we are making progress," it said. "However, insecurity, abject poverty, weak indigenous capacity, and widespread corruption create challenges for spending money. High staff turnover, pressure from the military, imbalances between military and civilian resources, unpredictable funding levels from Congress, and changing political timelines have further complicated efforts."
Among the problems cited by the senators was the use of U.S. contractors.
It said the United States relies heavily on such contractors but noted a lack of robust oversight. Much of U.S. aid goes not to the Afghan government but into the hands of international companies.
"This practice can weaken the ability of the Afghan state to execute its budget, lead to redundant and unsustainable donor projects, and fuel corruption," the report said.
But Carney defended the use of contractors. "Civilian assistance is important but it represents a small proportion of the overall cost of our mission in Afghanistan," he said. "But it is an essential component of our critical national security strategy in Afghanistan."