ROME - Defence Secretary Ash Carter says the Defence Department "deeply regrets the loss of innocent lives" from a U.S. military airstrike on a hospital in Afghanistan.

Responding to what officials said was an Afghan request, a U.S. gunship on Saturday fired on a medical clinic run by Doctors Without Borders in Kunduz. Afghans are battling to recapture the city from the Taliban. More than 20 people have died from the airstrike and dozens were wounded.

Carter issued a statement from Rome, saying the U.S. military "takes the greatest care" to prevent civilian casualties "and when we make mistakes we own up to them. That's exactly what we're doing right now."

Earlier Tuesday, the top commander in Afghanistan told Congress the airstrike was a mistake.