Doctors Without Borders is calling for an independent fact-finding mission under rules of the Geneva Conventions into a U.S. air strike on a hospital in Kunduz, Afghanistan that killed at least 22 people.

The medical aid group, known by its French language acronym MSF, said its call would mark the first time such a fact-finding mission would be launched under the Conventions.

The group's international president, Joanne Liu, told reporters Wednesday that the weekend strike "was not just an attack on our hospital, it was an attack on the Geneva Conventions. This cannot be tolerated."

MSF says the measure is a first step, aimed to establish facts about the incident and the chain of command that led to the strike. It is not aimed to press criminal charges just yet.