Medecins Sans Frontieres' president said she has received President Barack Obama's apology over the U.S. military strike on the charity's hospital in Kunduz, Afghanistan, but still wants an independent investigation. "We reiterate our ask that the U.S. government consent to an independent investigation led by the International Humanitarian Fact-Finding Commission to establish what happened in Kunduz, how it happened, and why it happened," said Dr. Joanne Liu, international president of the group. Obama called Liu Wednesday to apologize for the strike, which the United States has called a tragic mistake, the White House said. The group, which believes Saturday's airstrike in Kunduz may have been a war crime, appealed to the U.S., Afghanistan and other countries to mobilize a little-known commission to look into the tragedy. The aid group says it above all wants to ensure respect for international humanitarian law after the most deadly airstrike in its history. A dozen MSF staffers and 10 patients were killed in the hospital airstrike amid fighting between Afghan government forces and Taliban rebels in the northeastern city. The U.S. military has already vowed to conduct an investigation and says the airstrike was a mistake. Liu called for an impartial and independent probe into the attack, "particularly given the inconsistencies in the U.S. and Afghan accounts of what happened over recent days. "We cannot rely on only internal military investigations by the U.S., NATO and Afghan forces," the MSF president said. White House spokesman Josh Earnest said Obama offered condolences to the group's staff and pledged a "transparent, thorough and objective accounting of the facts." "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. The International Humanitarian Fact-Finding Commission, based in Geneva, is made up of diplomats, legal experts, doctors and some former military officials from nine European countries, including Britain and Russia. Created after the Gulf War in 1991, the commission has never deployed a fact-finding mission. The strike "was not just an attack on our hospital, it was an attack on the Geneva Conventions. This cannot be tolerated," she told reporters Wednesday. The U.S. airstrikes have all but shattered the humanitarian aid response in Kunduz, causing MSF, whose hospital was the primary medical facility in the region, and other aid groups to suspend operations there.