President Barack Obama on Sunday condemned the alleged slaughter of 16 Afghan civilians by an American soldier as "tragic and shocking" and assured Afghan President Hamid Karzai by telephone that the United States will "hold fully accountable anyone responsible" as the bloody incident plunged already deteriorating ties into a fresh crisis.
Obama called Karzai, who denounced the reported shooting rampage as an unforgivable "assassination," to make clear "his Administration's commitment to establish the facts as quickly as possible and to hold fully accountable anyone responsible," the White House said in a statement.
Obama expressed "his shock and sadness" to Karzai at the incident, which news reports said left mostly women and children among the dead, and "reaffirmed our deep respect for the Afghan people and the bonds between our two countries," the White House said.
Ahead of the call, Obama received a briefing on the incident from top aides including National Security Adviser Tom Donilon, Deputy National Security Advisor Denis McDonough, and Special Assistant to the President for Afghanistan and Pakistan Doug Lute, according to National Security Council deputy spokeswoman Caitlin Hayden.
Obama also issued a public statement saying he was "deeply saddened" by the reports and throwing his full support behind promises from Defense Secretary Leon Panetta and General John Allen, who commands NATO-led forces in Afghanistan, for a full investigation. Allen said in a statement that the US service member alleged to have carried out the attack was in custody.
"This incident is tragic and shocking, and does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan. I fully support Secretary Panetta's and General Allen's commitment to get the facts as quickly as possible and to hold accountable anyone responsible," Obama said.
The shooting rampage immediately reignited a debate over whether to hasten the US troop withdrawal from Afghanistan, where American-led forces are scheduled to hand over security to their hosts by the end of 2014. A new ABC News/Washington Post poll found US public sentiment against the war at near-record highs.
"I offer my condolences to the families and loved ones of those who lost their lives, and to the people of Afghanistan, who have endured too much violence and suffering," said the president.
The tragedy came just days after Obama and Karzai spoke by video conference and agreed that tensions had "calmed" since the burning of Muslim holy books on a US base.
That incident had sparked violent protests in Afghanistan and the killing of six US service members at the hands of their Afghan counterparts. It also led Obama to apologize for the desecration of the Qurans in an attempt to quiet clashes that he said endangered US military and civilian personnel. Republican presidential candidates, notably Newt Gingrich, condemned the apology.
Panetta said in a statement that he had spoken to Karzai to offer "my deepest condolences" and that "a full investigation is already underway."
"I gave President Karzai my assurances that we will bring those responsible to justice. We will spare no effort in getting the facts as quickly as possible, and we will hold any perpetrator who is responsible for this violence fully accountable under the law," said the defense secretary.
"I condemn such violence and am shocked and saddened that a U.S. service member is alleged to be involved, clearly acting outside his chain of command," Panetta said.
Panetta said the United States remained "steadfast in our resolve to work hand in hand with our Afghan partners."
"I am fully committed to ensuring that our cooperation continues. It is essential to forging a more peaceful future for the citizens of both our nations," he added.
Allen, who heads the International Security Assistance Force (ISAF), offered his "profound regret and deepest condolences" and promised "a rapid and thorough investigation" as well as medical care for those injured in the shooting.
"We will maintain custody of the U.S. service member alleged to have perpetrated this attack. And we will cooperate fully with local Afghan authorities as we ascertain all the facts," Allen said in a statement.
"This deeply appalling incident in no way represents the values of ISAF and coalition troops or the abiding respect we feel for the Afghan people. Nor does it impugn or diminish the spirit of cooperation and partnership we have worked so hard to foster with the Afghan National Security Forces," Allen said.
At a press conference last week, Obama had said that "the situation with the Quran burning concerns me. I think that it is an indication of the challenges in that environment, and it's an indication that now is the time for us to transition." He did not, however, say he would change his withdrawal timetable.
But calls to speed up the US draw-down could come from Afghanistan as well as from the American public, which has deeply soured on the conflict.
A narrow majority, 54 percent, of respondents in a new ABC News / Washington post poll say US troops should withdraw on time whether or not Afghan forces are self-sufficient.
And the survey -- which was conducted on Saturday, before the reported killing spree — found that 60 percent of Americans say the war has not been worth fighting. That's just four points shy of the record 64 percent who said the same thing one year ago.