President Barack Obama called the head of Doctors Without Borders yesterday to apologize for a U.S. airstrike that killed 12 aid workers in Afghanistan, as the relief group stepped up its calls for an independent inquiry into an attack it has labeled a possible war crime.

White House press secretary Josh Earnest said Obama spoke with Doctors Without Borders president Joanne Liu and expressed his condolences for the Saturday strike against a hospital in northern Afghanistan.

"When we make a mistake, we are honest, own up to it and apologize," Earnest told reporters at the White House. "The Department of Defense goes to great lengths to prevent civilian casualties, but in this case, there was a mistake, and it's one that the United States owns up to."

At least 22 people, including the 12 Doctors Without Borders staff, were killed early Saturday when a U.S. AC-130 gunship launched repeated attacks on the facility in the city of Kunduz. Afghan forces are battling there to oust Taliban fighters who overran the city a week ago.

Earnest said Obama promised a transparent and thorough investigation "and if necessary will implement changes to make sure tragedies like this one are less likely in the future."

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Earnest declined to say whether the White House would support demands from the aid group for an independent probe of the incident. The Pentagon, NATO and the Afghan government are conducting their own investigations into the attack.

Speaking to reporters in Geneva earlier in the day, Liu said the proposed commission would gather evidence from the United States, NATO and Afghanistan. After that, the charity would decide whether to seek criminal charges for loss of life and damage.

"If we let this go, we are basically giving a blank check to any countries at war," she said.

On Tuesday, Gen. John F. Campbell, the top commander for U.S. and allied countries in Afghanistan, told Congress that the U.S. aircraft, often used to support Special Operations forces, acted in response to a request from Afghan troops facing a Taliban attack. He said the hospital was "mistakenly struck."

But numerous questions remain about how the strike, in which an AC-130 gunship conducted repeated bombing raids on a building housing the hospital's emergency rooms and intensive care unit, could have happened.