KABUL -- U.S. forces may have mistakenly bombed a hospital in northern Afghanistan on Saturday, killing at least 19 people, including three children, in an incident that will probably raise new questions about the scope of American involvement in the 14-year war.

In a statement, Doctors Without Borders said an airstrike "partially destroyed" their trauma hospital in Kunduz, where the Afghan military has been trying to drive Taliban fighters from the city.

The airstrike killed at least 12 Doctors Without Borders staff members, the group said. Three children also were killed. At least 37 other people were seriously injured, including 19 staff members and 18 patients and caretakers. Officials warned the death toll could rise as dozens of people remain unaccounted for.

"This attack is abhorrent and a grave violation of International Humanitarian Law," Meinie Nicolai, the group's president, 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.' "

"We are deeply shocked by the attack, the killing of our staff and patients and the heavy toll it has inflicted on Kunduz," said Bart Janseens, director of operations for the hospital. "We do not yet have final casualty figures, but our medical teams are providing first aid and treating injured patients and . . . accounting for the deceased."

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In a statement, Defense Secretary Ashton Carter said: "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."

The United Nations' top human rights official called for an independent investigation, while equating the airstrike on the hospital to a war crime.

"This event is utterly tragic, inexcusable, and possibly even criminal," Zeid Ra'ad al-Hussein, the UN High Commissioner for Human Rights, said in a statement.