A U.S. airstrike in Kunduz, in Afganistan  killed at least 19 people at a hospital run by international medical-aid organization Médecins Sans Frontières (Doctors Without Borders) early Saturday.

MSF said 12 Afghan staff members and at least seven patients, among them three children, were killed and 37 people more, including patients and medical staff, were wounded.

The organization said it had informed all parties involved in the conflict of the precise location of the hospital, most recently on Tuesday. It said the airport was directly hit by a series of bombings at roughly 15 minutes intervals that continued for over an hour even after American and Afghan officials in Kabul and Washington were informed.

“This attack is abhorrent and a grave violation of international humanitarian law,” Meinie Nicolai, the president of MSF, said in a statement. “We demand total transparency from coalition forces. We cannot accept that this horrific loss of life will simply be dismissed as ‘collateral damage.’”

U.S. Army General John Campbell, who commands U.S. troops in Afghanistan, on Saturday called Afghan President Ashraf Ghani to apologize for the incident, according to a statement from Mr. Ghani’s office.