A US soldier in Afghanistan has shot dead 15 civilians and wounded others after entering their homes in Kandahar province, Afghan and Nato sources say.
He reportedly left his base early in the morning to attack village homes. Nine children are among the dead.
The White House voiced “deep concern” and Nato-led forces in Afghanistan promised a rapid inquiry.
Afghan President Hamid Karzai has condemned the attack and demanded an explanation from Washington.
In Kandahar’s Panjwai district, local people have gathered near the base to protest about Sunday’s killings, and the US embassy is advising against travel to the area.
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.
The unnamed soldier, thought to be a staff sergeant, is reported to have walked off his base at around 03:00 local time (22:30 GMT Saturday), then made his way to the nearby villages of Alkozai and Najeeban.
A local resident, Abdul Baqi, told the Associated Press news agency the soldier had apparently opened fire in three different houses.
“When it was happening in the middle of the night, we were inside our houses,” he said. “I heard gunshots and then silence and then gunshots again.”
In one house in Najeeban, the gunman reportedly killed 11 people, setting fire to their bodies before he left.
A relative of the 11 victims, Haji Samad, told Reuters news agency chemicals had been poured over the bodies and set alight.
“I saw that all 11 of my relatives were killed, including my children and grandchildren,” he added, weeping.
An unnamed woman witness in Najeeban said she had heard gunfire at about 02:00. A barking dog was shot dead by the gunman, she said.
She added that the Taliban had not been seen in the area for five months.
At least three of the child victims were killed by a single shot to the head, the BBC’s Quentin Sommerville reports from Kabul.
Photographs from the scene showed bodies, some of them clearly young children, placed in a vehicle under blankets.
Some reports suggested that more than one soldier was involved in the attack, and a statement by the Taliban accused Afghan security forces of playing a role.
A delegation from the provincial governor’s office has arrived in the village to determine exactly what happened, a spokesman said.
The soldier – who reportedly suffered a breakdown before the attacks – is said to have handed himself over to the US military authorities after carrying out the killings.