President Barack Obama attends a conversation co-hosted by Coworker.org during the White House Summit on Worker Voice.

United States President Barack Obama has apologized to Doctors Without Borders on Wednesday for the American air attack that killed at least 22 people at its hospital in Afghanistan, and said the U.S. would examine military procedures to look for better ways to prevent such incidents.

Obama’s telephone call to the group’s international president, Joanne Liu, came a day after the White House had stopped short of an apology, waiting to learn more while acknowledging that the attack was a U.S. mistake. White House spokesman Josh Earnest said Obama offered condolences to the staff and pledged a “transparent, thorough and objective accounting of the facts.”

Liu confirmed the apology and, in a written statement, reiterated her organization’s call for the U.S. government to consent to an independent investigation “to establish what happened in Kunduz, how it happened, and why it happened.”

“When the United States makes a mistake, we own up to it, we apologize where appropriate, and we are honest about what transpired,” Earnest said. He described Obama’s apology as “heartfelt.”

Emerging details about the erroneous strike have only fueled growing condemnation by Doctors Without Borders and other aid groups in the four days since the clinic in the northern city of Kunduz came under heavy fire that killed a dozen staffers and 10 patients.

After initial confusion, officials determined the U.S. had carried out the strike, an admission that complicates delicate U.S. efforts in Afghanistan as Obama weighs how many troops to leave there.

Investigations by the U.S., NATO and the Afghan government are underway, but the medical aid group has called them insufficient and has appealed to the U.S., Afghanistan and other countries to mobilize a commission to look into the tragedy. Without addressing that demand, the White House offered assurances that the Pentagon would dutifully carry out its internal probe.

Doctors Without Borders, known by its French language acronym MSF, has also said the strike may have been a war crime and has described it as an attack on the Geneva Conventions governing humanitarian treatment during war. The White House urged patience while the Pentagon works to establish what transpired.