Ambassador Karl Eikenberry said he found comments from "some" Afghan leaders "hurtful and inappropriate", according to a transcript of a speech released late on Sunday.
Although Eikenberry did not mention Karzai by name, the speech appeared to be a direct response to a string of verbal broadsides against Western troops serving in Afghanistan and the diplomatic and aid programmes that accompany them.
In one recent fiery speech Karzai warned that foreign soldiers risked being seen as occupiers because of civilian casualties they caused. Last week he said the West was polluting the country with weapons containing toxic chemicals.
Eikenberry said those comments left him ashamed and speechless in front of the relatives of US war dead.
"When I hear some of your leaders call us occupiers, I cannot look at these mourning parents, spouses, and children in the eye and give them a comforting reply," Eikenberry told an audience of students and academics at Herat University in western Afghanistan.
"When we hear ourselves being called occupiers and worse, our pride is offended and we begin to lose our inspiration to carry on," he added, in a personal addendum to a speech on education and transition.
Karzai's office declined to comment immediately comment on Eikenberry's statement.
He was speaking as US President Barack Obama mulls how steep a US troop drawdown pledged to start in July should be.
That will coincide with the first phase of a gradual handover of security control to the Afghan police and army, who are due to take responsibility for all of Afghanistan by the end of 2014, though critics warn this date is premature.
At present Nato is rushing to expand and train up security forces that have long struggled with problems ranging from widespread illiteracy, drug abuse and corruption to a dearth of leaders and equipment and a damaging rate of attrition.
Although the training team says progress is impressive, it will still be years before they have a real hope of holding off disciplined and battle-hardened insurgents across the country.
Even when they can fight alone, the size of the security forces and Afghanistan's sickly economy means they will need help paying salaries and buying equipment for years to come.
Eikenberry warned that patience to help Afghanistan seek security would not be infinite if Afghan partners were dismissive of US sacrifices of lives and money.
"At the point your leaders believe that we are doing more harm than good ... especially at a time our economy is suffering and our needs are not being met, the American people will ask for our forces to come home," Eikenberry said.
"I would ask, as the outgoing Ambassador, that your leaders please bear this in mind when they speak of my nation, my armed forces, and my people, as well as those others who also are making contributions to and sacrifices for your country."
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