The massacre of 16 villagers by a rogue United States soldier has triggered angry calls for an immediate American exit from Afghanistan as Washington tries to negotiate a long-term presence to keep the country from sliding into chaos again.
"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"
- US President Barack Obama
Afghanistan's parliament condemned the killing said the Afghan people had run out of patience with their lack of oversight of foreign soldiers in the country.
"The Wolesi Jirga [parliament] announces that once again Afghans have run out of patience with the arbitrary actions of foreign forces," the parliament said in a statement.
The Taliban vowed revenge against the US in a statement posted on their website on Monday that said "American savages'' committed the "blood-soaked and inhumane crime'' in Panjwai district.
Just days before Sunday's attack, Kabul and Washington had made significant progress in negotiations on a Strategic Partnership Agreement that would allow American advisers and special forces to stay in Afghanistan after foreign combat troops leave at the end of 2014.
But securing a full deal may be far more difficult now after the shooting spree in villages in the southern province of Kandahar, the Taliban heartland, which killed mostly women and children.
"This could delay the signing of the Strategic Partnership Agreement," an Afghan government official told the Reuters news agency.
The attack, the latest American public relations disaster in Afghanistan, may be a turning point for the United States in a costly and unpopular war now in its eleventh year.
Popular fury over the killing spree, which brought demands that the United States withdraw earlier than scheduled, could be exploited by the Taliban to gain new recruits.
"We have benefited little from the foreign troops here but lost everything - our lives, dignity and our country to them,"said Haji Najiq, a Kandahar shop owner.
"The explanation or apologies will not bring back the dead. It is better for them to leave us alone and let us live in peace."
Anti-Americanism, which boiled over after copies of the Muslim holy book, the Quran, were inadvertently burned at a NATO base last month is likely to deepen after the Kandahar carnage.
"The Americans said they will leave in 2014. They should leave now so we can live in peace," said Mohammad Fahim, 19, a university student. "Even if the Taliban return to power our elders can work things out with them. The Americans are disrespectful."
The civilian deaths may also force Afghan President Hamid Karzai to harden his stand in the partnership talks to appease a public already critical of his government's performance.
Barack Obama, the US president has called his Afghan counterpart, Hamid Karzai, to express his condolences after a US soldier in Afghanistan wandered off base and killed more than a dozen villagers in their homes.
The US has also promised a full investigation into Sunday's rampage in Kandahar, in southern Afghanistan, which left 16 civilians, who included three women and nine children, dead.
"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," Obama said in a statement released on Sunday by the White House.
Separately, Leon Panetta, the US defence secretary, pledged to "bring those responsible to justice" and said a full investigation was under way.
The US embassy in Kabul has sent out an alert to its citizens in Afghanistan cautioning that as a result of the shooting, "there is a risk of anti-American feelings and protests in coming days".
For his part, Karzai condemned the killings as "unforgivable" and demanded an explanation from the US government.
"When Afghan people are killed deliberately by US forces this action is murder and terror and an unforgivable action," he said in statement.
The shootings come at a particularly sensitive and critical time for the US, just as violence over the burning of copies of the Quran, the Muslim holy book, at a US base was starting to calm down.
At least 41 people were killed in violence related to those protests.
Senior US officials were scrambling to determine what caused the soldier to go on a shooting spree after leaving his base in southern Afghanistan, apparently heavily armed and carrying night-vision equipment.
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Officials confirmed that the soldier was being detained in Kandahar and that the military was treating at least five wounded people.
One US official said the soldier, an army staff sergeant, was believed to have acted alone and that initial reports indicated he returned to the base after the shooting and turned himself in.
General John Allen, the US military commander in Afghanistan, issued a statement pledging a "rapid and thorough investigation" into the incident, and said the soldier will remain in US custody.
Al Jazeera's Bernard Smith, reporting from Herat on Sunday, said the soldier entered three houses near the base and opened fire on civilians.
"We are now being told by the police sources that the US soldier left his base at three o'clock this morning. It would have been pitch-black wherever he walked," he said.
"The soldier went through three separate houses, shooting at people as they slept in their beds. After the soldier shot these people, he turned himself in.
"It is frankly disastrous. It is not just a disaster for the people who were murdered and killed in their houses, it is disaster for the country I suspect."
Najeeb Azizi, a Kabul-based Afghan analyst, said the shooting will have deep repercussions on the already tenuous relations with the US.
“It is a very tragic incident in particular because the Afghan and US governments are trying to sign a strategic agreement for a long term," he said.
"A very bad message the Afghan people are getting, that if US military remains in Afghanistan beyond 2014 and their attitude and behaviour remains the same - of killing innocent civilians - what will be the consequences, and how will the Afghan people respond to it."