UN High Commissioner for Human Rights, Zeid Ra'ad al Hussain said the bombing of an Afghan hospital in Kunduz by the US may constitute a war crime.
The US-led air strike on the hospital in the Taliban-held city of Kunduz killed at least 19 people, including 12 staff of Doctors Without Borders.
Mr Hussain said the attack was "inexcusable" and possibly criminal, and called for a full and transparent investigation, saying that "if established as deliberate in a court of law, an air strike on a hospital may amount to a war crime".
US defence chief Ash Carter described the incident as "tragic" and acknowledged that US forces were operating nearby in support of the Afghan army.
"While we are still trying to determine exactly what happened, I want to extend my thoughts and prayers to everyone affected. A full investigation into the tragic incident is underway in coordination with the Afghan government."
Afghan President Ashraf Ghani's office said Army General John Campbell, head of the US-led forces in Afghanistan, had apologised for the incident.
MSF officials said they "frantically phoned" NATO and Washington DC, as the attack went on for "nearly an hour".
In an interview with Al Jazeera, Bart Janssesns, MSF operations chief, demanded "more answers" from the US government, adding that the location of the hospital has been well-known by US-led forces for the last five years.