U.S. officials have launched an investigation after 12 local staff members of Doctors Without Borders and at least seven patients, three of them children, were killed after an explosion near their hospital in the northern Afghan city of Kunduz that may have been caused by a nearby airstrike.

In a statement, the international charity said the "sustained bombing" took place Saturday at 2:10 a.m local time. Afghan forces backed by U.S. airstrikes have been fighting to dislodge Taliban insurgents who overran Kunduz on Monday.

At least 37 other people were seriously injured--19 staff members and 18 patients and caretakers, the organization said. Dozens were missing, raising concerns the death toll could rise.

A senior defense official told Fox News on Saturday that the Taliban have been in control of the area around the hospital since Monday, guarding the building and drawing U.S. special operations forces into a firefight in the area. U.S. forces called in the airstrike because they were under fire and needed cover, the official said.

Taliban fighters were among those being treated at the hospital, a defense official told Fox News.

While defense officials told Fox News they “regret the loss” of innocent life, they say the incident could have been avoided if the Taliban had not used the hospital as a base, and the civilians there as human shields.

A defense official also confirmed to Fox News Saturday that a 1-star U.S. military general has been dispatched from Kabul to Kunduz to investigate the bombing of the hospital.

Defense Secretary Ash Carter called the bombing a "tragic incident."

"While we are still trying to determine exactly what happened, I want to extend my thoughts and prayers to everyone affected," he said in a statement. "A full investigation into the tragic incident is underway in coordination with the Afghan government."

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