KABUL, Afghanistan  President Hamid Karzai acknowledged on Monday that he regularly receives bags of cash from the Iranian government in payments amounting to millions of dollars, as evidence mounted of a worsening rift between his government and its American and NATO supporters.

During an often hostile news conference, Mr. Karzai also accused the United States of financing the “killing” of Afghans by paying private security contractors to guard construction projects and convoys in Afghanistan. He has declined to postpone a December deadline he set for ending the use of private security forces despite urgent pleas from Western organizations, including development organizations, that need protection here.

His statements were the latest indication that American relations with Mr. Karzai were badly frayed, despite diplomatic efforts to mend ties and improve governance in Afghanistan. The tensions threaten to undermine President Obama’s goal of handing responsibility for the war against the Taliban to Mr. Karzai and the Afghan military, allowing the United States to begin withdrawing troops next year.

“They do give us bags of money  yes, yes, it is done,” Mr. Karzai said, responding to questions about a report in The New York Times on Sunday that Iran sends regular cash payments to his chief of staff, Umar Daudzai. “We are grateful to the Iranians for this.”