Saying "even war has rules," an official of the international humanitarian-aid organization Médecins Sans Frontières (MSF), or Doctors Without Borders, said the "mistaken" US airstrike on its hospital in Kunduz, Afghanistan, last week was "an attack on the Geneva Conventions."

The air attack killed 22 people inside the hospital and wounded scores of others.

At a press conference in New York City on Wednesday, Jason Cone, US executive director of MSF, said, "Today we say, enough. Even war has rules."

The MSF is now calling for an independent investigation by a UN commission into how the attack happened, TIME reported.

MSF officials said such an investigation "would be the most appropriate way to get a full accounting of the facts."

The officials reiterated that sentiment in a statement following reports of the formal apology tendered by US President Barack Obama to MSF president Joanne Liu.

In his telephone conversation with Liu, Obama vowed to conduct a full investigation into the aerial bombings apparently launched by US military forces on an MSF facility in Afghanistan, the White House said.

"Based on what the President has learned, he believed it was appropriate to do what we've done before...to own up to our mistakes and to vow to carry out a full investigation to get the bottom of what exactly happened," White House Press Secretary Josh Earnest said during a press briefing on Wednesday, according to TIME.

The White House has yet to complete investigation into the case, said Earnest.

The US Department of Defense and NATO are also reportedly launching separate investigations into the incident.

US military officials had previously acknowledged that the airstrike was a mistake and had expressed "deep regrets'' for the lost lives.

On Saturday, the US Department of Defense announced that it will seek to make "condolence payments" to the families of victims of a US air strike.

Obama also reportedly called on President Ashraf Ghani of Afghanistan to express condolences for the death of Afghan civilians.

"The US military takes the greatest care in our operations to prevent the loss of innocent life, and when we make mistakes, we own up to them. That's exactly what we're doing right now," US Defense Secretary Ash Carter said in a statement.

"We will do everything we can to understand this tragic incident, learn from it, and hold people accountable as necessary," he said.