A US soldier in Afghanistan has killed 10 civilians and wounded five in Kandahar province after suffering a breakdown, officials say.
He left his military base in the early hours of the morning and opened fire after entering local homes; women and children are said to be among the dead.
Nato said it was investigating the "deeply regrettable incident".
Anti-US sentiment is already high in Afghanistan after US soldiers burnt copies of the Koran last month.
US officials have apologised repeatedly for the incident at a Nato base in Kabul, but they failed to quell a series of protests and attacks that killed at least 30 people and six US troops.
Local people have reportedly gathered near the base in Panjwai district to protest about Sunday's killings, and the US embassy is advising against travel to the area.
The soldier has not been named, but is thought to be a staff sergeant.
He walked off his base at around 03:00 local time (22:30 GMT Saturday).
According to a resident quoted by Associated Press, he opened fire in three separate houses in the village of Alkozai.
"I heard gunshots and then silence and then gunshots again," Abdul Baqi said.
Local tribal leaders said women and children were among the dead.
The BBC's Quentin Somerville in Kabul says the death toll could rise as high as 17.
A delegation from the provincial governor's office has arrived in the village to determine what has happened, a spokesman said.
After carrying out the killings, the soldier reportedly handed himself over to the US military authorities.
The Nato-led International Security Assistance Force (Isaf) said in a statement that US officials in Afghanistan would work with their Afghan counterparts to investigate what happened.
"This is a deeply regrettable incident and we extend our thoughts and concerns to the families involved," Isaf added.
Meanwhile, in Kabul, Afghan President Hamid Karzai said his government still expects to sign a strategic partnership with the United States in the next couple of months.
In a televised speech, he said discussions would continue on the precise role the US will play in Afghanistan after Nato hands over security responsibility to Kabul at the end of 2014.
On Friday, Kabul and Washington reached a deal to transfer US-run prisons in the country to Afghan control.