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The statements came amid claims by Afghan officials that the Obama administration offered to write the letter as part of an effort to keep a small number of U.S. troops in Afghanistan well past the 2014 deadline to withdraw.

"No such letter has been drafted or delivered. There is not a need for the United States to apologize to Afghanistan," National Security Adviser Susan Rice said on CNN's "Situation Room."

"Quite the contrary, we have sacrificed and supported them in their democratic progress and in tackling the insurgents and al Qaeda. So that (letter of apology) is not on the table."

Rice said she has seen news reports but has no idea where they are coming from, describing the claims as a "complete misunderstanding of what the situation is."

The claim first surfaced following talks Tuesday between U.S. Secretary of State John Kerry and Afghan President Hamid Karzai, when Karzai's spokesman told reporters that Kerry said President Barack Obama would write an official letter guaranteeing the United States would not repeat "past mistakes" that led to civilian casualties.

Asked later about such a letter, State Department spokeswoman Jen Psaki said, "I don't have anything to tell you about whether there will or won't be a letter."

The conversation between Karzai and Kerry came ahead of a meeting of Afghan leaders -- known as a Loya Jirga -- to potentially sign off on a deal that would allow a limited number of American troops to stay to train Afghan security services.

Kerry declined an invitation to attend to the Loya Jirga, but offered to write a letter to the leaders offering U.S. assurances about a future security relationship, said two senior U.S. officials, who spoke on condition of anonymity.

The letter Kerry offered would also clarify the U.S. position on past issues, such as civilian casualties and a pledge to do everything possible to avoid them, said one of the officials.

But the letter in no way would be offered as an apology, both officials said.

White House spokesman Jay Carney said a "general agreement" between the two countries was reached last month. There have been talks in the weeks since "to finalize the text," he said.

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Rice said the United States was in the last stages of negotiations on a framework that Kerry and Karzai signed off on last month during talks.

"Without such an agreement, that level of cooperation would be all but impossible."

Karzai's office issued a statement spelling out some points of contention, including the Afghan government's opposition to raids "on any Afghan homes by U.S. forces," while Washington wants to be able to conduct such operations "in exceptional circumstances."

According to the statement, Karzai proposed two alternatives:

-- Have Kerry himself make the case for military raids when the Loya Jirga convenes later this week

-- Don't sign any bilateral security agreement until a new Afghan government takes shape after the upcoming presidential election, set for next April