·Obama asked revisions of options on a pending buildup of U.S. forces in Afghanistan.

·The White House said Obama's options have been narrowed down to four.

·Media reports said the options ranged from sending 10,000 to 40,000 additional  troops.

    WASHINGTON, Nov. 12 (Xinhua) -- President Barack Obama has asked his top aides to refine the four options they presented to him on a pending buildup of U.S. forces in Afghanistan.

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    The president was "pushing for revisions to clarify how and when U.S. troops would turn over responsibility to the Afghan government," a senior administration official was quoted by CNN as saying Thursday, speaking on condition of anonymity.

    At the eighth meeting with his war council Wednesday, Obama raised questions about the four options, the official said.

    Obama's concerns "could alter the dynamic of both how many additional troops are sent to Afghanistan and what timeline would be for their presence in the war zone," he said.

    "The key sticking points appear to be timeline and mounting questions about the credibility of the Afghan government," the official said.

    The president was also trying to establish an exit strategy before committing to sending a significant number of U.S. troops, he added.

    The White House said earlier this week that the president's options on Afghan troop buildup have been narrowed down to four options, without giving any details.

    U.S. media reports said the recommendations ranged from sending 10,000 to 40,000 additional U.S. troops.