[Update: Story updated 3:30 p.m. Eastern time.]

From Geneva, Switzerland, officials from Doctors Without Borders called for an independent fact-finding mission to investigate the US air strike on a hospital in Kunduz, Afghanistan, that killed at least 22 people. Invoking international humanitarian law under the Geneva Conventions, the inquiry would gather facts and evidence from the United States, Afghanistan, and NATO and then make a decision on charging the US for war crimes.

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."

President Obama personally called Dr. Liu to apologize for the incident and extend his condolences for the attack, White House spokesman Josh Earnest said on Wednesday. When asked whether Mr. Obama offered any explanation for the incident, Earnest said no. "He merely offered his heartfelt apology" and a commitment to find out what went wrong.

Liu said the medical aid group, known by its French name Medecins Sans Frontieres, is "working on the assumption of a possible war crime," but said the group's true aim is to establish facts about the incident and the chain of command, and clarify the rules of operation for all humanitarian organizations that operate in conflict zones.

MSF says the importance of the call is to ensure that humanitarian law is respected, and clear up rules of operation for all humanitarian organizations that operate in conflict zones.

"The US attack on the MSF hospital in Kunduz was the biggest loss of life for our organization in an airstrike," Liu said. "Tens of thousands of people in Kunduz can no longer receive medical care now when they need it most. Today we say: enough. Even war has rules."

The group said it sent letters to 76 countries on Tuesday that have signed Article 90 of the additional protocol to the Geneva Conventions and are awaiting responses. The organization is seeking to put a 15-member commission of independent experts that was set up in 1991 into action. In this case, the US and Afghanistan – which are not signatories – must also consent to such a commission.

MSF said its call would mark the first time such a fact-finding mission would be commissioned under the Conventions.

No responses have yet been given from the US or any other countries, according to the organization. US Army Gen. John Campbell testified before the Senate Armed Services Committee on Tuesday, where he said the strike on the hospital was unintentional.

"The decision to provide aerial fires was a US decision, made within the US chain of command," he said. "The hospital was mistakenly struck. We would never intentionally target a protected medical facility."

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He promised a "thorough, objective, and transparent" investigation into the incident. He also said Afghan forces had requested air support from the US, having been in combat with Taliban forces in Kunduz.

This report contains material from Reuters and the Associated Press.