US General John W. Nicholson, the new commander of American and NATO forces in Afghanistan, has apologized for last year’s US bombing of a hospital that killed dozens of Afghan civilians.

The now-closed hospital, run by the international medical charity Medecins Sans Frontieres (MSF) in the northern Afghan city of Kunduz, was attacked by a US Air Force AC-130 gunship on October 3, one of the most lethal military aircraft in the world. The bombing killed 42 people and left 37 others injured.

The Geneva-based charity said the incident "constitutes a war crime" and demanded an international investigation, but none has been undertaken.

On Tuesday, General Nicholson met family members of victims and the staff of the hospital in Kunduz to express his condolences.

"As commander, I wanted to come to Kunduz personally and stand before the families, and people of Kunduz, to deeply apologize for the events" that led to the bombing, Nicholson said.

"I grieve with you for your loss and suffering; and humbly and respectfully ask for your forgiveness," added Nicholson.

According to US military officials, more than a dozen American military personnel have been punished for bombing the MSF hospital, but they face no criminal charges.

The military officials, who spoke on condition of anonymity, said last week the punishments are largely administrative, such as letters of reprimand and ending chances for further promotion.

The officials, who were not authorized to discuss the issue publicly, said the disciplinary process is nearly complete.

The Pentagon has previously said some personnel were suspended from their duties but has given no further details.

US Army General John Campbell, who was the top American commander in Afghanistan at the time of the bombing, has since relinquished command. Campbell has called the attack a "tragic but avoidable accident caused primarily by human error."

Medecins Sans Frontieres, known as Doctors Without Borders in English, has publicly questioned the argument that the US strike was a mistake.

In this AFP photograph taken on November 10, 2015, staff members walk through the damaged Doctors Without Borders hospital in Afghanistan.

The merciless strike, which lasted for more than an hour, led to the closure of the hospital, depriving tens of thousands of Afghans of vital medical care.

According to an investigation by the medical charity, US military commanders continue to attack the hospital for 17 minutes after being warned that their aircraft was firing on a medical center full of doctors and civilians.

In November 2015, the US military claimed the crew of the AC-130 gunship had been dispatched to hit a Taliban command center in a different building, 411 meters away from the hospital.

However, the crew was hampered by problems with their targeting sensors that led them to begin firing at the hospital even though they saw no hostile activity there.