A US service member has killed 16 people - including nine children and three women - in an Afghanistan shooting spree condemned by President Hamid Karzai as "an assassination".
President Karzai demanded an explanation from the United States, adding new tensions to a relationship already severely strained over Americans burning Muslim holy books on a base in Afghanistan.
The burnings sparked violent protests and attacks that left some 30 people dead. Six US service members have been killed in attacks by their Afghan colleagues since the Koran burnings came to light, but the violence had started to calm down.
"This is an assassination, an intentional killing of innocent civilians and cannot be forgiven," Mr Karzai said in a statement. He said he has repeatedly demanded the US stop killing Afghan civilians.
Senior US officials are scrambling to determine what caused the American Army soldier to leave his base and carry out the shooting.
Officials confirmed that a soldier was being detained in Kandahar and that the military was treating at least five wounded. One US official said the soldier, an Army staff sergeant, was believed to have acted alone.
President Barack Obama later offered his condolences to the families of those allegedly shot by the US soldier.
In a statement released by the White House, Obama said the attack was "tragic and shocking".
He also vowed "to get the facts as quickly as possible and to hold accountable anyone responsible".