The airstrike that struck the hospital in Afghanistan on Saturday, killing 22 people, was not called in by U.S. troops, the general in charge of U.S. forces in Afghanistan said Monday.

Afghan forces in the northern Afghan city of Kunduz came under fire and requested support from a U.S. AC-130 gunship, which then opened fire on a hospital run by Doctors Without Borders, Gen. John Campbell told reporters Monday morning.

Campbell would not go into further details about Saturday's attack, but said U.S., NATO and Afghan officials all are conducting investigations into what happened.

"If errors were committed we will acknowledge them and hold accountable those responsible," he said.

Campbell promised to be transparent with the investigation and said he expects results of a preliminary report within days.

Campbell said initial reports that the strike was launched to support U.S. troops who had come under Taliban fire were incorrect.

"I think the impression that people got after the first couple days is they were firing directly on U.S. forces and what I'm telling you today is as I've talked to the investigating officer, as we continue to get updated information, that that was not the case," he said.

U.S. troops are embedded as advisers with Afghan troops and "retain their inherent right of self defense," Campbell said. He would not say how close U.S. troops were to the fighting. Afghan troops, backed by U.S. advisers, are fighting to retake Kunduz from the Taliban, which attacked the city Sept. 28.

"Unfortunately, the Taliban have decided to remain in the city and fight from within, knowingly putting civilians at harm," Campbell said.

He said he had not suspended or made changes to the train, advise and assist program in Afghanistan.

Campbell will testify Tuesday at a Senate Armed Services Committee hearing on the situation in Afghanistan.

Doctors Without Borders, also known as Médecins Sans Frontières, released a statement Sunday calling for an independent investigation of the strikes, which they call "a grave violation of International Humanitarian Law."

"Under the clear presumption that a war crime has been committed, MSF demands that a full and transparent investigation into the event be conducted by an independent international body. Relying only on an internal investigation by a party to the conflict would be wholly insufficient," the statement said.

Twelve doctors and 10 patients, including three children, were killed in the attack, which the group says specifically targeted the hospital.

"We reiterate that the main hospital building, where medical personnel were caring for patients, was repeatedly and very precisely hit during each aerial raid, while the rest of the compound was left mostly untouched," the statement said.