WASHINGTON — The hugely expensive U.S. attempt at nation-building in Afghanistan has had limited success and may not survive an American withdrawal, according to the findings of a two-year congressional investigation.

The report calls on the administration to rethink urgently its assistance programs as President Barack Obama prepares to begin drawing down U.S. troops in Afghanistan this summer.

Ryan Crocker, Obama’s choice for U.S. ambassador to Afghanistan, said Wednesday the United States must continue its investment there to prevent it from backsliding into a safe haven for terrorists.

The report, prepared by the Senate Foreign Relations Committee’s Democratic majority staff, describes the use of aid money to stabilize areas the military has cleared of Taliban fighters — a key component of the administration’s counterinsurgency strategy — as a short-term fix that provides politically pleasing results. But it says that the enormous cash flows can overwhelm and distort local culture and economies, and there is little evidence the positive results are sustainable.

One example it cites is the Performance-Based Governors Fund, authorized to distribute up to $100,000 a month to provincial leaders for use on local expenses and development projects. In some provinces, “this amount represents a tidal wave of funding” that local officials are incapable of “spending wisely.”

Because oversight is scanty, the report says, the fund encourages corruption. Although the U.S. plan is for the Afghan government to eventually take over this and other programs, it has neither the management capacity nor the funds to do so.

The report also warns that the Afghan economy could slide into a depression with the inevitable decline of the foreign military and development spending that now provides 97 percent of the country’s gross domestic product.

At his confirmation hearing, Crocker told the Senate Foreign Relations Committee that the goal is “good enough governance” in Kabul, “good enough to ensure that the country doesn’t degenerate into a safe haven for al-Qaida.

“We’re not out to, clearly, create a shining city on a hill,” Crocker said.

The career diplomat, who also served in Beirut, Islamabad and Baghdad, said the killing of Osama bin Laden was an important step toward disrupting al-Qaida, but much work remains to ensure al-Qaida never threatens the U.S. from Afghanistan again.

Crocker said progress in Afghanistan has been significant but is still fragile and reversible. The U.S. is expected to begin withdrawing troops from Afghanistan next month.

Committee Chairman John Kerry, D-Mass., told Crocker, “Our current commitment, in troops and dollars, is neither proportional to our interests nor sustainable, in my judgment.”

The report says “single most important step” the administration could take is to stop paying Afghans “inflated salaries” — often 10 or more times as much as the going rate — to work for foreign governments and contractors. Such practices, it says, have “drawn otherwise qualified civil servants away from the Afghan government and created a culture of aid dependency.”

The report says that even when U.S. development experts determine that a proposed project “lacks achievable goals and needs to be scaled back,” the U.S. military often takes it over and funds it anyway.

It also cites excessive use and poor oversight of contractors.

All U.S. development projects in Afghanistan should be re-examined,the report adds, to determine whether they are “necessary, achievable, and sustainable.”

The Washington Post, Associated Press and New York Times contributed to this article.