By Yeganeh Torbati

WASHINGTON (Reuters) - Afghan forces asked for U.S. air support while fighting the Taliban in Kunduz shortly before an air strike resulted in the deaths of civilians there, the American commander of international forces in Afghanistan said on Monday.

U.S. Army General John Campbell's comments fell short of squarely acknowledging U.S. responsibility for an air strike that killed 22 people in an Afghan hospital run by aid group Medecins Sans Frontieres (MSF) on Saturday.

MSF on Monday reiterated its call for an independent investigation into the incident, saying discrepancies in the accounts of what happened made such a probe "ever more critical."

"We have now learned that on October 3 Afghan forces advised that they were taking fire from enemy positions and asked for air support from U.S. forces," Campbell said in a briefing with reporters. "An air strike was then called to eliminate the Taliban threat, and several civilians were accidentally struck."

Campbell said U.S. forces were not under direct fire in the incident and the air strike had not been called on their behalf, contrary to previous statements from the U.S. military.

In a statement, MSF General Director Christopher Stokes said Campbell's comments amounted to trying to pass responsibility for the strike to the Afghan government.

"The reality is the U.S. dropped those bombs," the statement said. "The U.S. hit a huge hospital full of wounded patients and MSF staff. The U.S. military remains responsible for the targets it hits, even though it is part of a coalition. There can be no justification for this horrible attack."

U.S. Army Brigadier General Richard Kim is the senior investigator on the incident and is in Kunduz now, Campbell said. He said the U.S. military will ensure transparency in investigating the incident, and that NATO and Afghan officials would conduct their own investigations as well.

"If errors were committed, we'll acknowledge them," Campbell said. "We'll hold those responsible accountable, and we'll take steps to ensure mistakes are not repeated."

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Responding to earlier calls by MSF for an international probe, Campbell said on Monday that there would be U.S., NATO, and Afghan investigations into the strike, but did not close the door to an outside investigation.

"If there's other investigations out there that need to go on, we'll make sure to coordinate those as well," Campbell said.

Campbell declined to comment on whether the United States had called a pause to air strikes, but said he had not suspended "train, advise, and assist" support from U.S. forces to the Afghans.

Campbell said he expected a preliminary report on the incident "very shortly, in the next couple of days."

(Reporting by Yeganeh Torbati; Editing by Bill Trott and Jonathan Oatis)