At least 22 people died in the Saturday morning strike, including 12 staffers, the charity's international president said Wednesday.
The organization is known internationally as Medecins Sans Frontieres, or by the acronym MSF.
Guilhem Molinie, its representative for Afghanistan, said that of the 461 staffers who were in the hospital at the time of the strike, there are still 24 missing. Of the patients, there are still nine missing.
"Our largest loss of life has occurred at the hands of American forces," he said.
President Obama called MSF international president Dr. Joanne Liu Wednesday to apologize for the strike.
MSF has called for an independent investigation into the fatal air strike, saying an outside group must get involved, "given the inconsistencies in the U.S. and Afghan accounts of what happened," Liu said Tuesday.
MSF General Director Christopher Stokes said that the charity's hospital was operating under the agreement of all parties involved in the conflict. He said that they had shared their GPS coordinates with military authorities and their staff and patients felt safe as a result.