A US soldier in Afghanistan has killed at least 16 civilians and wounded five after entering their homes in Kandahar province, senior local officials say.
He left his military base in the early hours of the morning and opened fire in at least two homes; women and children were 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.
Lt Gen Adrian Bradshaw, deputy commander of Nato-led forces, said he was unable to "explain the motivation behind such callous acts", adding that "our thoughts and prayers are with those caught in this tragedy".