GENEVA — Doctors Without Borders called for an independent and unprecedented fact-finding mission on Wednesday to investigate a U.S. airstrike on a hospital run by the medical aid group in Afghanistan that killed at least 22 people.

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, also known by its French language acronym MSF, says it above all wants to ensure respect of 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 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.

MSF international president Joanne Liu called for an impartial and independent probe of the facts and circumstances of 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,” she said.

MSF wants to mobilize the International Humanitarian Fact-Finding Commission, based in the Swiss capital of Bern.