An apparent U.S. airstrike wreaked havoc in the embattled Afghan city of Kunduz early Saturday, killing at least 19 civilians and seriously wounding dozens of others at a hospital run by the medical nonprofit Doctors Without Borders.

The bombing, which U.S. military sources said targeted Taliban fighters in the proximity of the hospital compound, prompted harsh condemnation from the United Nations and human rights groups.

In a statement, Army Col. Brian Tribus, a spokesman for U.S. Forces in Afghanistan, said, “U.S. forces conducted an airstrike in Kunduz city… against individuals threatening the force. The strike may have resulted in collateral damage to a nearby medical facility.”

According to Doctors Without Borders, blasts from sustained bombing repeatedly struck the hospital, severely damaging the facility, northeastern Afghanistan’s only trauma center.

“The bombing continued for more than 30 minutes after American and Afghan military officials in Kabul and Washington were first informed” that bombs were hitting the hospital, the group said in a statement.

Doctors Without Borders said it had informed U.S. forces of the hospital’s GPS coordinates on multiple occasions, the most recent of which was Tuesday.

“On Sept. 29 and the weeks prior, we had made sure that at the highest levels of both the military and civilian chain of command in the U.S. government that they had the GPS coordinates of our compound,” Doctors Without Borders USA Executive Director Jason Cone told PBS NewsHour.

Secretary of Defense Ashton Carter offered his condolences and promised an investigation.

“A full investigation into the tragic incident is underway in coordination with the Afghan government,” Carter said in a statement, noting that “U.S. forces in support of Afghan Security Forces were operating nearby, as were Taliban fighters.”

The United Nations High Commissioner for Human Rights condemned the incident and said the strike may constitute a war crime.

“This deeply shocking event should be promptly, thoroughly and independently investigated and the results should be made public,” Zeid Ra’ad al-Hussein said, according to a Reuters report. “The seriousness of the incident is underlined by the fact that, if established as deliberate in a court of law, an airstrike on a hospital may amount to a war crime.”

The Kunduz attack constitutes a grave violation of International Humanitarian Law.We demand a full & transparent account from the Coalition — MSF International (@MSF) October 3, 2015

Doctors Without Borders’ Jason Cone called the incident “one of the darkest days in our organization’s history” and demanded accountability for the strike.

“It was clear that this was a fully functioning hospital — that there were hundreds of patients and staff inside — and we need answers,” Cone told PBS NewsHour. “We will accept nothing less than a full, transparent, independent investigation.”

Other aid groups also strongly condemned the strike.

In an online statement, Jean-Nicolas Marti, head of the International Committee of the Red Cross delegation in Afghanistan, said, “This is an appalling tragedy. Such attacks against health workers and facilities undermine the capacity of humanitarian organizations to assist the Afghan people at a time when they most urgently need it.”

As part of a larger offensive across northern Afghanistan, Taliban forces captured Kunduz, Afghanistan’s sixth-largest city, on Monday.

Fighting in the city has intensified as Afghan government troops attempt to retake the provincial capital. U.S. forces have conducted roughly a dozen airstrikes in support of Afghan troop, the AP reported.

According to Doctors Without Borders, 12 staff members and at least seven patients were killed in the bombardment, including three children.

At the time of the strike Saturday, there were 105 patients and their care-takers in the hospital — part of 394 people treated at the hospital since fighting began — as well as over 80 staff members, the group said. Some of those present during the bombing are still missing.

The strike on the hospital will likely compromise access to trauma care in the region as fighting between Taliban and coalition forces intensifies.

“You’re basically seeing the complete loss of access to healthcare for trauma injuries in the area. There’s no one to replace it, and our hospital is obviously not operational under the circumstances,” Cone said.

Doctors Without Borders is transferring some critically injured patients to a hospital in Pul-i-Kumri, two hours’ drive from the crippled hospital in Kunduz.