US President Barack Obama has phoned his Afghan counterpart Hamid Karzai to express condolences over the massacre of 16 villagers in Kandahar.
Nine children were killed along with adults in their homes when a US soldier based locally allegedly went on a gun rampage during the night.
Obama vowed to hold accountable anyone responsible for the "tragic and shocking" incident.
President Karzai has condemned the attack and demanded an explanation.
The unnamed suspect, who had apparently recently suffered a mental breakdown, returned to his base in Kandahar's Panjwai district after the attack and handed himself into custody.
Earlier, Gen John R Allen, commander of the Nato-led International Security Assistance Force in Afghanistan, said US officials there would work with their Afghan counterparts to investigate what had happened.
The US embassy in Kabul is warning of possible anti-American reprisals.