Afghan security deal clouded by dispute over U.S. admission of "mistakes"

* Karzai spokesman: Obama to issue letter about war mistakes

* But White House says no such letter in work

* Tribal elders approval needed to put pact into motion (Adds Rice comment, senior State Department official)

By Hamid Shalizi and Lesley Wroughton

KABUL/WASHINGTON, Nov 19 (Reuters) - Last-minute efforts to finalize a security pact between the United States and Afghanistan were clouded on Tuesday by differences over whether President Barack Obama had agreed to issue a letter acknowledging U.S. mistakes made during the 12-year war.

The Afghan government said it received assurances that such a letter would be provided this week to a grand council of Afghan elders. But Susan Rice, Obama's national security adviser, insisted that such an offer - which would draw criticism from Republicans and anger American war veterans - is "not on the table."

A spokesman for Afghan President Hamid Karzai said an Obama letter was part of talks on a long-sought security pact that would allow a residual force of U.S. troops to stay in Afghanistan beyond 2014.

Karzai and U.S. Secretary of State John Kerry overcame the main stumbling blocks to an agreement in a telephone call on Tuesday but the State Department said some issues still had to be resolved before a final draft can be presented to the Loya Jirga, a gathering of Afghan tribal and political leaders that will meet in Kabul starting on Thursday.

Aimal Faizi, Karzai's spokesman, said the two sides agreed on provisions giving U.S. troops immunity from Afghan law and allowing them to enter Afghan homes in exceptional circumstances, something the Afghan president had resisted.

Faizi said the accord - which must now be approved by the Loya Jirga - was partly due to a promise that Obama would give a written admission of U.S. military errors in a war that has claimed many civilian casualties in addition to combatants.

"Both sides agreed that Obama send a letter ... assuring the president and the people of Afghanistan that the right to enter into Afghan homes by U.S. forces and the extraordinary circumstances will not be misused," Faizi said in Kabul.

"The whole idea of having a letter was to acknowledge the suffering of the Afghan people and the mistakes of the past. That was the only thing that satisfied the president," Faizi added.

But Rice denied any such U.S. intention.

"No such letter has been drafted or delivered," she told CNN. "There is not a need for the United States to apologize to Afghanistan. Quite the contrary. We have sacrificed and supported them in their democratic progress and in tackling the insurgency and al Qaeda."