President Barack Obama on Wednesday morning called the head of Doctors Without Borders to apologize for a U.S. airstrike that hit one of the international humanitarian organization's hospitals in northern Afghanistan, killing 12 medical staff and 10 patients.

"This morning from the Oval Office, President Obama spoke by telephone with Doctors Without Borders International President Dr. Joanne Liu to apologize and express his condolences for the MSF [Médecins Sans Frontiéres] staff and patients who were killed and injured when a U.S. military airstrike mistakenly struck an MSF field hospital in Kunduz, Afghanistan over the weekend," White House Press Secretary Josh Earnest told reporters during the daily briefing.

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The apology came as Liu on Wednesday called for an international investigation into the deadly incident, saying it was "unacceptable" that the bombing could be dismissed as collateral damage or a simple mistake.

Army Gen. John Campbell, the top U.S. commander in Afghanistan, spoke about the attack during a Senate hearing on Tuesday. He said Afghan forces called for an airstrike, but the decision to provide it was made by the U.S. and that a hospital was “mistakenly struck.”

Talking to reporters on Wednesday, Earnest said Obama assured Liu that the Defense Department's investigation would offer a "transparent, thorough and objective" accounting of the facts and that he would follow through on any necessary changes.

Earnest also said Obama had called Afghan President Ashraf Ghani to express his condolences.