President Barack Obama called the president of Medicins Sans Frontieres (MSF) on Wednesday to apologise for the US bombing of a hospital in Afghanistan that left 22 people dead.
MSF has said the air strike that levelled the hospital constituted a war crime, and has demanded an independent investigation.
The US has admitted it mistakenly bombed the MSF facility, in the battleground city of Kunduz, last Saturday.
The White House said on Tuesday that additional information was needed before Mr Obama would issue an apology, but reversed course the next day.
Josh Earnest, Mr Obama's spokesman, said Mr Obama called Dr Joanne Liu, MSF's international president, "to apologise and express his condolences for the MSF staff and patients who were killed and injured".
"The president assured Dr Liu that the department of defence investigation currently underway would provide a transparent, thorough and objective accounting of the facts and circumstances of the incident," Mr Earnest added.
Mr Obama's apology came a day after the US military commander in Afghanistan acknowledged US responsibility for the bombing during a Senate hearing.
"To be clear, the decision to provide aerial flyers was a US decision made within the US chain of command," Gen John Campbell said on Wednesday. "A hospital was mistakenly struck. We would never intentionally target a protected medical facility."
MSF has said that all parties fighting in Kuduz had been made aware of the location of the hospital.
Speaking to reporters prior to the call from Mr Obama, Dr Liu said MSF insisted on an independent investigation because it "cannot rely on the internal military investigations by the US, Nato, and Afghan forces".
"If we let this go, we are basically giving a blank check to any countries at war," she said.
The air strike came amid efforts to drive the Taliban out of Kunduz, a city of 300,000 in north-western Afghanistan.
Mr Earnest, the White House spokesman, said the US had been transparent and forthright in the aftermath of the air strike.
"The United States, when we make a mistake, we're honest about it, we own up to it, we apologise where necessary, as the president did in this case and we implement the kinds of changes that make it less likely that those kinds of mistakes will occur in the future," he said.
Mr Obama also called Ashraf Ghani, Afghanistan president, to express his condolences for the innocent Afghan lives lost in the strike.