GENEVA (AP) — Doctors Without Borders called for an independent and unprecedented fact-finding mission on Wednesday to investigate a U.S. airstrike on its hospital 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, 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, Bern. It is made up of diplomats, legal experts, doctors and at least three former military officials from nine European countries, including Britain and Russia.

Fully created after the Gulf War in 1991, the commission has never deployed a fact-finding mission.

Liu said MSF is "working on the assumption of a possible war crime," but said its real goal is to establish facts about the incident and the chain of command, and clear up the rules of operation for all humanitarian organizations in conflict zones.

The weekend 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 their operations there.

MSF, a Nobel Peace Prize-winning organization that provides medical aid in conflict zones, is awaiting responses to letters it sent Tuesday to 76 countries that signed Article 90 of the additional protocol to the Geneva Conventions, seeking to mobilize the 15-member commission. The Conventions, whose roots date to 1864, lay out rules on the conduct of armed conflict, notably over protecting noncombatants — including civilians, aid workers, medical personnel and prisoners of war.

For the commission to be mobilized, a single country would have to call for the fact-finding mission, and the U.S. and Afghanistan — which are not signatories — must also give their consent for one to go forward. MSF says it has had no response yet from the U.S. or any other countries.

Helen Durham, director of international law and policy at the International Committee of the Red Cross, said it would be a positive development if the commission could help "clarify the facts surrounding this tragic incident." She said she believed a commission investigation could complement separate ones conducted by the U.S., NATO and Afghanistan.

The office of Afghan President Ashraf Ghani didn't immediately respond to a request for comment. In the United States, the Pentagon, State Department and White House have all said that they do not believe an international investigation is needed.

MSF legal director Francoise Saulnier said it was "difficult to activate politically something that puts countries face to face" and would require good will. However, she noted the importance of setting a precedent in application of humanitarian law, and ensuring that the never-used commission — which could act in any number of conflict zones in the world today — has a role in that.

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Lynne O'Donnell in Kabul, Afghanistan, contributed to this report.