KABUL, Afghanistan  One evening last August, as President Hamid Karzai wrapped up an official visit to Iran, his personal plane sat on the airport tarmac, waiting for a late-running passenger: Iran’s ambassador to Afghanistan.

The ambassador, Feda Hussein Maliki, finally appeared, taking a seat next to Umar Daudzai, Mr. Karzai’s chief of staff and his most trusted confidant. According to an Afghan official on the plane, Mr. Maliki handed Mr. Daudzai a large plastic bag bulging with packets of euro bills. A second Afghan official confirmed that Mr. Daudzai carried home a large bag of cash.

“This is the Iranian money,” said an Afghan official, who spoke on condition of anonymity. “Many of us noticed this.”

The bag of money is part of a secret, steady stream of Iranian cash intended to buy the loyalty of Mr. Daudzai and promote Iran’s interests in the presidential palace, according to Afghan and Western officials here. Iran uses its influence to help drive a wedge between the Afghans and their American and NATO benefactors, they say.