US President Barack Obama on Wednesday apologized to Doctors without Borders (MSF) for a deadly US air strike on an Afghan hospital, as the medical charity demanded an international probe into the incident.

US President Barack Obama on Wednesday apologized to Doctors without Borders (MSF) for a deadly US air strike on an Afghan hospital, as the medical charity demanded an international probe into the incident.

Three separate investigations -- by the US military, NATO and Afghan officials -- are currently under way into Saturday's catastrophic strike in the northern Afghan city of Kunduz, which left 22 people dead.

 The US military has offered a series of shifting explanations for the bombing raid, from initially talking about "collateral damage" to now admitting, as Obama did in his call to MSF chief Joanne Liu, that the strike was a mistake.

One report said the top US and NATO commander in Afghanistan thought American forces had broken their own rules of engagement in carrying out the strike, which sparked international outrage.

Obama called Liu to "apologize and express his condolences for the MSF staff and patients who were killed and injured when a US military airstrike mistakenly struck an MSF field hospital in Kunduz," White House spokesman Josh Earnest said.

The president assured Liu that the Pentagon probe would "provide a transparent, thorough and objective accounting of the facts and circumstances of the incident," Earnest said.

Obama also called Afghan President Ashraf Ghani to express his condolences, and commended the "bravery" of Afghan forces battling to secure the city of Kunduz from Taliban fighters.

But the charity, which condemned the attack as a war crime, stressed the need for an international inquiry, saying the bombing raid was in contravention of the Geneva Conventions.

"We cannot rely on an internal military investigation," Liu told reporters in Geneva, insisting that the International Humanitarian Fact-Finding Commission should probe the bombing.

"This was not just an attack on our hospital -- it was an attack on the Geneva Conventions," Liu said.

"This cannot be tolerated."

MSF's US chief Jason Cone later called on Obama to consent to the commission, which he said would "send a powerful signal of the US government commitment to... international humanitarian law and the rules of war."