NATO-led forces in Afghanistan have been placed on alert and the Taliban has vowed revenge in the aftermath of a shooting rampage by a suspected U.S. soldier that left 16 Afghan villagers dead.
But a White House spokesman has said the killings will not change U.S. strategy or objectives in Afghanistan.
U.S. President Barack Obama telephoned his Afghan counterpart, Hamid Karzai, and said he was "shocked" and "deeply saddened" at news of the tragedy. He promised to hold accountable anyone responsible for the March 11 predawn rampage in southern Kandahar Province.
Obama told Karzai that the shooting spree does not reflect the respect the United States has for the Afghan people.
Karzai has described the shootings as the "intentional killing of innocent civilians" and said they cannot be forgiven.
The unnamed U.S. soldier -- reportedly a staff sergeant -- is said to have walked off his base before dawn on March 11 and gone house-to-house in the Alekozai village of Kandahar's Panjwai district.
Nine children and at least three women were among the dead.
In addition to killing 16 people, the soldier is reported to have burned the bodies of some of the victims.
AP quoted U.S. officials as saying the soldier who carried out the shooting had served three tours in Iraq and was on his first deployment in Afghanistan.
U.S. Defense Secretary Leon Panetta assured Karzai by telephone that a full investigation is under way.
Afghanistan's parliament has demanded that the United States punish the culprits and put them on public trial.
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Calling U.S. soldiers "sick-minded," the Taliban has vowed to avenge the massacre.
The U.S. Embassy in Kabul has warned American personnel in Afghanistan to be on guard against possible reprisal attacks.
In a statement, the Taliban promised the families of the victims that it would take revenge "for every single martyr with the help of Allah."
U.S. presidential spokesman Jay Carney said on March 12 that the U.S. and NATO timetable to hand over security responsibilities to the Afghans remains on schedule for 2014. Carney added that discussions about the pace of the drawdown have been ongoing with U.S. allies and will be on the agenda at the NATO meeting in Chicago in May.
Meanwhile, speaking in New York on the sidelines of a UN Security Council meeting on Syria, Secretary of State Hillary Clinton said the United States "is committed to seeing that those responsible are held accountable."
"Let me say that like many Americans, I was shocked and saddened by the killings of innocent Afghan villagers this weekend," Clinton said. "We send our condolences to families who have lost their loved ones and to the people of Afghanistan. This is not who we are, and the United States is committed to seeing that those responsible are held accountable."
NATO Secretary-General Anders Fogh Rasmussen added his voice to those expressing shock and sadness over the killings. In a written statement, the NATO chief offered condolences to family members of the victims, backed an investigation, and said NATO remains committed to working with its Afghan partners to build a “strong and stable Afghanistan.”
ISAF spokesman General Carsten Jacobsen denied claims by some residents of Kandahar's Panjwai district that more than one soldier was involved in the killing spree. Some reports have quoted Afghan villagers as saying they believe more than one soldier could have been responsible.
"We were shocked and saddened to hear of the shooting incident yesterday in Kandahar Province," Jacobsen said. "ISAF offers its profound regret and deepest condolences to the victims and their families. We pledge to all the noble people of Afghanistan our commitment to a rapid and thorough investigation."
Afghanistan's parliament on March 12 demanded the United States “punish the culprits and try them in a public trial before the people of Afghanistan." It said the Afghan people had “run out of patience with the arbitrary actions of foreign forces."
The incident comes as the U.S. and Afghan governments were reportedly moving closer to agreeing on a new strategic pact covering the long-term presence of U.S. troops in Afghanistan following the planned withdrawal of foreign combat forces at the end of 2014.
Afghan parliament member Shagul Rezai has warned that the shooting could impact those negotiations.
"This kind of incident will have a negative impact and will create distrust between the people and coalition forces," she said. "And, of course, it will have a negative impact on the ongoing negotiations with regards to [the U.S.-Afghan] strategic partnership [agreement]."
The killings also follow a wave of unrest that erupted after the burning of Korans by U.S. personnel at the Bagram military base near Kabul. The February 20 burning -- which U.S. officials said was a mistake -- sparked six days of violent anti-American protests in which more than 30 people were killed, including six U.S. soldiers.
With AFP, dpa, and Reuters reports