The White House said on Monday that the United States is concerned about Iran's trying to have "a negative influence" on Afghanistan, following reports that Iran regularly gives large sums of cash to senior aid to Afghan President Hamid Karzai.

White House Deputy Press Secretary Bill Burton told reporters that Iran should have a positive influence on the formation of Afghan government.

Burton urged Iran to ensure that Afghanistan will not be the safe haven for terrorists and the origin of future terrorist attacks.

The New York Times reported on Saturday that Karzai's chief of staff, Umer Daudzai, was covertly receiving "bags of cash," totaling millions of U.S. dollars, from Iranian officials.

The story said the money is used by Afghan leaders as "slush fund" to pay lawmakers and tribal leaders to secure their loyalty to the Karzai government. And Iran intends to use the money to increase its influence in the war-torn country.

Earlier the day, U.S. State Department spokesman Phillip Crowley said that the United States still remains skeptical of Iran's motives in Afghanistan.

On Monday in Kabul, Karzai acknowledged receiving regular cash payments from Iran but insisted that the payment is transparent. He told reporters that the United States gives cash to some of his offices as well.

When asked if U.S. President Barack Obama is aware that the U.S. is giving out cash to Afghan officials or offices, Burton refused to get into the specifics.

He only said that President Obama, from the first day he was in office, has stepped up efforts to fight corruption in Afghanistan, and he will continue to do so.

However, Crowley admitted on Monday that some U.S. assistance has been given to Afghanistan "in the form of cash." He argued that the U.S. concern is not about "the form of support" but "what the assistance is for."