President Hamid Karzai of Afghanistan furiously demanded an explanation from Washington on Sunday for a killing spree by a US soldier that claimed the lives of 16 Afghan villagers, including nine children and three women.
President Barack Obama last night telephoned Karzai to tell him that those responsible for the attack would be brought to account.
"President Obama extended his condolences to the people of Afghanistan, and made clear his administration's commitment to establish the facts as quickly as possible and to hold fully accountable anyone responsible," said a White House statement. "The President reaffirmed our deep respect for the Afghan people and the bonds between our countries."
Obama had earlier described the incident as "tragic and shocking" and said it did not "represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan".
Karzai said: "This is an assassination, an intentional killing of innocent civilians and cannot be forgiven."
Western officials said the attack was unprecedented in the conflict. It risks further inflaming anti-Western sentiment, already high after personnel at a US airbase near Kabul last month apparently inadvertently put copies of the Koran in a rubbish incinerator, sparking violence across the country. Two American officers were shot dead.
After yesterday's incident, the US embassy warned its citizens to brace for "anti-American feelings and protests in coming days".
US military officials were unable to offer any explanation why the unnamed army sergeant walked off his base in the Panjwayi district at around 3am and went to the villages of Balandi and Alkozai, about 500 yards away.
He entered three houses and opened fire, killing 11 from a single family in one house. Many of the bodies were also burned, neighbours said. Photographs from the scene showed bodies, some of them clearly young children, placed in a vehicle under blankets.
"No Taliban were here. No gun battle was going on," one woman told a news agency. "We don't know why this foreign soldier came and killed our innocent family members. Either he was drunk or he was enjoying killing civilians."
US military officials in Kabul denied reports from villagers that more than one soldier had been involved in the attack.
A spokesperson for the coalition said: "It was one US service member acting alone. This was not part of any operation. He walked out of the compound and fired on civilians, then returned to his compound and handed himself in."
Karzai sent an investigating team to the area, as did Nato.
The shooting is the latest in a series of public relations crises for the coalition that have added to widespread resentment against foreign troops.
As well as the burning of the Korans, video was released of US Marines urinating on dead Taliban fighters. The killing of civilians in air strikes continues to weaken Afghan public support for Karzai's international backers. The Taliban posted a statement calling yesterday's killings "genocidal". It said: "The so-called American peacekeepers have once again quenched their thirst with the blood of innocent Afghan civilians in Kandahar province."
Western officials said it was unclear what violence the killings could provoke.
"We don't know if they are going to get in their minibuses tomorrow after the funerals and head for a huge demonstration in Kandahar," said one official. "It depends a lot on what the local officials and mullahs say."
Two years ago, a group of American soldiers was accused of deliberately targeting Afghan civilians in Kandahar province during patrols and keeping body parts as trophies of their victims.