State Department spokesman Philip Crowley told reporters that Washington did not question Iran's right to provide aid to Afghanistan, nor Afghanistan's right to receive it.
"What we think is important is Afghans having the ability to shape their own future without negative influences from its neighbours," he said.
Crowley added that it is up to the Afghan government to explain how it spends the money it receives from other countries."But we remain skeptical of Iran's motives, given its history of playing a destabilising role with its neighbours," he said.
In Kabul, Afghan President Hamid Karzai admitted yesterday that his chief of staff had received "bags of money" from Iran but insisted the payment was transparent and a form of aid from a friendly country.
The New York Times reported Saturday that Karzai's chief of staff, Umar Daudzai, received regular cash payments from Iran.
When asked if the United States feared influence peddling, Crowley replied that "we would hope that that money would be used for the kind of constructive institution building" in which the international community is engaged.
"It is something that we have talked to the Afghan government about," Crowley said.
He also said that Washington has at times given Kabul aid in "the form of cash," but much of the aid is "now provided electronically" through "carefully vetted" financial institutions.
Crowley said that the US government has "strong systems of accountability in place" to ensure the money is used for the benefit of the Afghan people.
He added that the massive flow of international aid to Afghanistan has "added to the challenge of corruption" in that country as Washington tries to strengthen what he called weak Afghan institutions of government.
A senior State Department official conceded that the image of Kabul receiving bags of money from Tehran was not a good one. "No question. These are not great optics," the official told reporters on the condition of anonymity.