President Hamid Karzai of Afghanistan launched a broadside against his coalition allies on Saturday, saying the motives behind their presence were suspect and even complaining that their weaponry is polluting his country.

Mr. Karzai denied earlier reports — including from his own statements — that his government was negotiating with Taliban leaders, but he said that the Americans were doing so. American officials have never publicly acknowledged such talks, and the Taliban have denied them categorically.