WASHINGTON -- The U.S. airstrike that killed 22 at a medical clinic in northern Afghanistan over the weekend was requested by Afghan forces who reported being under Taliban fire, and was not sought by U.S. forces, the top commander of American and coalition forces in Afghanistan said Monday.

Gen. John F. Campbell made the statement at a hastily arranged Pentagon news conference. He said he was correcting an initial U.S. statement that said the airstrike had been in response to threats against U.S. forces.

"We have now learned that on Oct. 3, Afghan forces advised that they were taking fire from enemy positions and asked for air support from U.S. forces," Campbell said. "An airstrike was then called to eliminate the Taliban threat and several civilians were accidentally struck. This is different from the initial reports which indicated that U.S. forces were threatened and that the airstrike was called on their behalf."

His revised account does not clarify whether the clinic was targeted in error or whether other mistakes may have been made by U.S. forces.

"If errors were committed we will acknowledge them," Campbell said.

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He declined to provide more details, saying a military investigation is ongoing. He said he learned from the investigator that it was the Afghans, not the Americans, who requested the airstrike.

Campbell, whose headquarters is in Kabul, was in Washington on Monday because he is testifying before two congressional committees this week.

Meanwhile, the medical charity Doctors Without Borders continued to press U.S. and Afghan officials for an independent investigation into the bombing early Saturday of its hospital in Kunduz, in which at least 22 people were killed.

Christopher Stokes, the general director of MSF, the charity's French acronym, said on Monday he was "disgusted by the recent statements coming from some Afghanistan government authorities justifying the attack."

The Afghan government has said that Taliban fighters were inside and shooting from the hospital, CBS News correspondent Mark Phillips reports. Vickie Hawkins, Executive Director of MSF U.K., told Phillips that those comments are "outrageous."

"They are to an extent justifying the destruction of a fully functioning hospital," Hawkins said.