* White House says not considering speeding up troop withdrawals
* Obama says U.S. will bring war to an end "responsibly"
* Decision on troop numbers to be taken at May NATO summit (Adds Obama quotes; incorporates AFGHANISTAN-USA/WITHDRAWAL)
By Jeff Mason
WASHINGTON, March 13 (Reuters) - President Barack Obama promised on Tuesday that a U.S. soldier's shooting rampage in Afghanistan would be investigated as if the dead were "our own citizens and our own children," and pledged a responsible end to the war in the country.
The White House said it was not reviewing options to speed up troop withdrawals from Afghanistan after the shooting and that a decision on troop numbers was not expected before a NATO summit in Chicago on May 20-21.
An unnamed U.S. soldier is accused of walking off his base in Afghanistan's Kandahar province early Sunday and killing at least 16 villagers, mostly women and children.
The incident has added to strains between the United States and Afghanistan when they have been trying to agree to a pact that would allow U.S. advisers and special forces to stay in Afghanistan after combat troops leave at the end of 2014.
Coming in the middle of a U.S. election year, the incident has also sparked more criticism of Obama's Afghan policy from Republican presidential candidates.
"The United States takes this as seriously as if it was our own citizens and our own children who were murdered," Obama told reporters at the White House, saying he had conveyed this message in a call to Afghan President Hamid Karzai.
"We're heartbroken over the loss of innocent life."
"The killing of innocent civilians is outrageous, it is unacceptable, it's not who we are as a country, and it does not represent our military," he said.
The Pentagon is conducting an investigation, and Obama added:
"I can assure the American people, and the Afghan people, that we will follow the facts wherever they lead us, and we will make sure that anybody who was involved is held fully accountable with the full force of the law."
The New York Times reported that the Obama administration was discussing whether to speed up the withdrawal of U.S. troops from Afghanistan by cutting at least an additional 20,000 of the current 90,000 troops by 2013.
White House spokesman Tommy Vietor said the White House was not currently studying options for the size of further troop withdrawals beyond a plan already underway to bring home 33,000 troops by mid-September.
"No decisions have been made," he said.
"After that initial reduction, our troops will continue coming home at a steady pace as Afghan security forces move into the lead. The president will make decisions on further drawdowns at the appropriate time based on our interests and in consultation with our allies and Afghan partners."
In the past, Republican presidential candidates Mitt Romney, Rick Santorum and Newt Gingrich have criticized Obama's withdrawal timeline, saying it emboldened U.S. enemies. After the shooting, however, Santorum and Gingrich suggested the United States should consider withdrawing earlier.
Most Americans are tired of the war and want U.S. troops to come home, but Obama said the United States would stick to its task.
"We will continue the work of devastating al Qaeda's leadership and denying them a safe haven," he said.
"There's no question that we face a difficult challenge in Afghanistan, but I am confident that we can continue the work of meeting our objectives, protecting our country and responsibly bringing this war to a close." (Additional reporting by Matt Spetalnick and Samson Reiny; Editing by David Brunnstrom)
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