The US commander of the Nato-led force in Afghanistan promised a "rapid and thorough investigation" into a killing spree by a US soldier that claimed 16 lives, including nine children.
"I am absolutely dedicated to making sure that anyone who is found to have committed wrong-doing is held fully accountable," General John Allen, the commander of the International Security Assistance Force, said.
Allen said he was "shocked and saddened" by the incident, which erupted early Sunday when a US soldier walked out of his base in the southern Kandahar province and opened fire on men, women and children.
An AFP reporter at the scene counted the bodies of 16 people after the shooting rampage. Afghan President Hamid Karzai said the victims included nine children and three women.
ISAF said a soldier had been arrested in connection with the incident but did not say how many people were killed or wounded.
US-Afghan tensions had already plunged to a new low last month when the discovery that American troops on a work detail had accidentally burned Korans ignited deadly violence and riots.