By ROBERT BURNS
AP National Security Writer
WASHINGTON (AP) - The Obama administration is only beginning to calculate the pace of troop withdrawals from Afghanistan beyond this summer, facing an endgame fraught with political risk and complicated by shocking setbacks like the alleged U.S. slaughter of Afghan civilians.
At stake is not only President Barack Obama's pledge to prevent Afghanistan from reverting to the terrorist haven it was before the Sept. 11, 2001, attacks, but also his commitment to wind down the war while crafting a long-term security relationship with the Afghans.
U.S. military commanders want to keep as many troops in the country as possible until the Dec. 31, 2014, target date for having all combat forces out. They fear a too-rapid pullout would risk surrendering the security gains they have made in recent years.
But the White House faces the prospect of intensifying political pressure to end the military mission, especially after events such as the burning of Muslim holy books by U.S. troops last month that triggered a wave of Afghan violence, including the killings of at least six U.S. troops by Afghan troops.
Rep. Adam Smith, D-Wash., a top Democrat on the House Armed Services Committee, is among those calling for a faster withdrawal.
"It is time to bring our troops home, and, while the president has laid out a responsible path to do so, we should continue to look for every opportunity to accelerate our timeline," Smith wrote in an opinion piece in USA Today.
Troop-cut planning is a product of complex calculations of the number and kinds of forces that are expected to be required in areas of Afghanistan with varying degrees of security and political weaknesses.
For example, it has already been decided that as many as 10,000 Marines will come out of Helmand province by September, reflecting vastly improved security in a former Taliban stronghold. At the same time, additional military resources are to be sent to eastern Afghanistan, where military gains have been more difficult and where the insurgency is more complex.
Obama said twice Tuesday that the war must end `'responsibly," following his warnings Monday that the U.S. must not rush to the exits and risk all it has done in Afghanistan. Just last week, he said the transition out must be gradual, "not a cliff." That language is similar to his approach in Iraq, where the last U.S. troops left in December 2011.
From today's total of 90,000, Obama has instructed his top commander in Afghanistan, Marine Gen. John Allen, to cut the number of U.S. troops to 68,000 by the end of September.
Beyond that, no decisions have been made, although officials are starting to sketch out possibilities, officials said Tuesday.
Allen's chief spokesman, Navy Capt. John Kirby, said Allen is under "no pressure" from the White House to come up with a plan for continuing the troop withdrawal beyond September. Allen has yet to present Defense Secretary Leon Panetta with his plan for getting down to the 68,000 mark.
"There is no work being done by Gen. Allen and his staff" to determine further troop reductions after he reaches the 68,000 plateau, Kirby said. "There are no demands for options or recommendations."
Other officials said, however, that informal work is being done in Washington now, ahead of the formal process of recommendations and discussions that would end with decisions by Obama on future troop cuts. White House officials have had preliminary discussions about how to reshape the Afghan force next year, when the U.S. wants to shift to a largely advisory role, a person familiar with the discussion said.
White House National Security Council advisers held at least one recent small meeting where possible withdrawal numbers and scenarios were discussed, the person said, including the possibility of bringing additional forces home in the closing months of 2012.
The person spoke on condition of anonymity to describe sensitive discussions.
Administration officials stressed that no formal options have been crafted.
"There is not a discussion about specific numbers or specific options at this time," White House spokesman Jay Carney said.
He noted that deliberations on further troop reductions must include NATO allies, since the overall withdrawal plan is a coalition commitment worked out at a conference in Lisbon in November 2010. NATO leaders are due to meet again in Chicago in May.
Obama has said additional reductions will ensue at a "steady pace," but no numerical markers have been set. The hope is that by mid-2013 the Afghans will take the lead security role throughout their country, enabling the U.S. and its allies to end their combat role.