By ROBERT BURNS
AP National Security Writer
WASHINGTON - 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.
"We should have been gone a long time ago," Sen. Joe Manchin, D-W.Va., said in an interview. "It's time to come home and rebuild America."
Senate Foreign Relations Chairman John Kerry, D-Mass., however, cautioned against using the weekend slayings, apparently by a U.S. soldier, of more than a dozen Afghan civilians as a reason to abandon the existing schedule for ending the combat mission.
"I still think you have to do this mission on the basis of what makes sense for the mission so you're not jeopardizing everything that has been achieved," Kerry said in an interview.
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 before the formal process begins that would end with decisions by Obama on future troop cuts.
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. U.S. and allied troops probably would stay beyond 2014 as military advisers rather than as combat forces.
The New York Times reported Tuesday that one of the options being considered is for at least 10,000 more to come home by the end of December, and then 10,000 to 20,000 more by June 2013.
In response, administration officials stressed that no formal options have been crafted. Said White House spokesman Jay Carney: "There are no options being reviewed with specific troop numbers attached to them. There are no individuals promoting specific options over others."
Outrage, both in the U.S. and in Afghanistan, over the recent killings by each side of their nominal allies has made it more difficult for Obama to strike a balance between "staying the course" and signaling that the war must end.
Obama tried to address that double-sided message in interviews with local television stations Monday. He said the latest events will not speed up the withdrawal, a message aimed mostly at European allies eager to bring their own forces home. But Obama appeared to reflect the frustration of a war-weary home front when he stressed that the war is ending.
"My plan calls for us to get out of Afghanistan, to transition to Afghan lead," he told KVUE-TV in Austin. "We need to do that responsibly, we don't want to do it in such a way that suddenly, everything that we have done over the last decade collapses. But it is time for us to go."
To KABC in Los Angeles, Obama said, "Keep in mind that I have put us on a path where we're going to have this war over by the end of 2014, that our troops will be coming out, but we'll be coming out responsibly."
Associated Press writers Lolita C. Baldor in Bishkek, Kyrgyzstan and Donna Cassata, Anne Gearan, Ben Feller and Julie Pace in Washington contributed to this report.