A suspected US air strike on a hospital in the Afghan city of Kunduz has killed 19 people, medical charity Doctors Without Borders says.
The UN has condemned the bombardment as "inexcusable, and possibly even criminal".
Dozens more were seriously wounded at the facility, a key lifeline that has been running "beyond capacity" during fighting that saw the Taliban seize control of the northern provincial capital for several days.
The strike early on Saturday (local time) left the building engulfed in flames, with photos posted by Doctors Without Borders showing their staff shocked and dazed.
"At 2:10 am local time ... the MSF trauma centre in Kunduz was hit several times during sustained bombing and was very badly damaged," the organisation, known by its French initials, said.
"Twelve staff members and at least seven patients, including three children, were killed; 37 people were injured. This attack constitutes a grave violation of international humanitarian law."
The charity said the bombing continued for more than 30 minutes after American and Afghan military officials were first alerted they were being hit.
"All parties to the conflict, including in Kabul and Washington, were clearly informed of the precise location (GPS coordinates) of the MSF facilities," the statement added.
NATO conceded US forces may have been behind the strike but has not so far commented on the specific claims of MSF, which has long treated the war-wounded from all sides of the conflict.
"The strike may have resulted in collateral damage to a nearby medical facility. This incident is under investigation."
The incident has renewed concerns about the use of US air strikes in Afghanistan, a deeply contentious issue in the 14-year campaign against Taliban insurgents.
UN rights chief Zeid Ra'ad Al Hussein called for a full and transparent probe, noting that, "if established as deliberate in a court of law, an air strike on a hospital may amount to a war crime."
"This event is utterly tragic, inexcusable and possibly even criminal," he said.
US Defense Secretary Ashton Carter said a "full investigation" is being carried out.
Obama extends condolences over Afghan hospital
US President Barack Obama has offered his "deepest condolences" over a suspected US airstrike on a hospital in the Afghan city of Kunduz that killed 19 people.
"On behalf of the American people, I extend my deepest condolences to the medical professionals and other civilians killed and injured in the tragic incident at a Doctors Without Borders hospital in Kunduz," he said in a statement released by the White House on Saturday.
"The Department of Defence has launched a full investigation, and we will await the results of that inquiry before making a definitive judgment as to the circumstances of this tragedy," Obama said.