WASHINGTON: US President Barack Obama telephoned the head of Medecins San Frontieres (MSF) and apologised for a deadly air strike on the aid group's hospital in Kunduz, Afghanistan, the White House said on Wednesday.

On the call with the medical charity's president, Joanne Liu, Obama also said the US investigation into the incident would “provide a transparent, thorough and objective accounting of the facts and circumstances of the incident,” White House spokesman Josh Earnest told reporters.

He added that, if necessary, the president would "implement changes to make tragedies like this one less likely to occur in the future."

Obama also called Afghan President Ashraf Ghani to express condolences for the lives lost of patients and staff during the strike, Earnest said.

MSF seeks independent inquiry into 'mistaken' attack

MSF, also known as Doctors Without Borders, is calling for an independent international fact-finding commission to be established to probe the bombing, which took place over the weekend.

The medical charity said that the inquiry would gather facts and evidence from the United States, Nato and Afghanistan, as well as testimony from MSF staff and patients who survived Saturday's attack.

Only then would MSF consider whether to bring criminal charges for loss of life and partial destruction of its trauma hospital, which has left tens of thousands of Afghans without access to health care, it said.

“If we let this go, as if was a non-event, we are basically giving a blank cheque to any countries who are at war,” MSF International President Joanne Liu told a news briefing.

“If we don't safeguard that medical space for us to do our activities, then it is impossible to work in other contexts like Syria, South Sudan, like Yemen.”

In New York, Jason Cone, executive director of MSF in the United States, called for the International Humanitarian Fact-Finding Commission to be activated for the first time since its 1991 creation under the Geneva Conventions.

Cone urged US President Barack Obama to consent to the commission.

“Doing so will send a powerful signal of the US government's commitment to and respect for international humanitarian law under rules of war,” Cone said at a news conference.

Liu spoke of the chaos as the bombs fell for an hour.

“Our patients burned in their beds, MSF doctors nurses, and other staff were killed as they worked. Our colleagues had to operate on each other,” she said.

The United States military took responsibility on Tuesday for the air strike that killed 22 people, including 12 MSF staff, calling it a mistake.

Read more: Attack on Kunduz hospital a mistake, says US

US Defense Secretary Ash Carter while speaking to reporters in Rome on Wednesday said, “We are conducting a full and transparent investigation and will make the findings of that investigation known as they are found and will hold accountable anyone responsible for conduct that was improper.”

The Afghan Ministry of Defence said on Sunday Taliban fighters had attacked the hospital and were using the building “as a human shield”, which the medical group denied, while pointing out it would be a war crime not to treat the wounded.

Earlier on Sunday, MSF said it had shut down operations in Kunduz after the fatal US air strike on its hospital and called for an independent investigation.

Read more: Medical charity leaves Kunduz after air strike on hospital