UN Secretary-General Ban Ki-moon has called for the punishment of those involved in the recent massacre of 18 Afghan civilians.
"We expect that an investigation will rapidly establish the facts and that those responsible will be held accountable," Reuters quoted Ban as saying in a statement on Tuesday.
"We will continue to urge all authorities and military personnel, who are engaging in these operations to pay extreme and utmost care to protect the civilian population," he added.
On Sunday, a US trooper opened fire on Afghan civilians inside their homes in the district of Panjwaii in Kandahar Province in southern Afghanistan, killing at least 18 civilians and injuring several others.
Earlier in the day, US President Barack Obama pledged a thorough investigation into the killings.
“The US take this as seriously as if it were our own citizens and our children, who were killed," Obama said.
Civilian casualties in Afghanistan have been a major source of tension between Kabul and Washington.
The US-led invasion of Afghanistan was launched in 2001. The attack removed the Taliban from power, but insecurity rages on across the country, despite the current presence there of thousands of foreign troopers.