U.S. Defense Secretary Leon Panetta apologized Tuesday, February 21 for the "inappropriate treatment" of copies of the Koran at an American base in Afghanistan and promised a swift investigation, AFP reported.

Panetta's statement came as protesters besieged the Bagram air base in Afghanistan shouting "Death to Americans" following reports that NATO troops had burned the Muslim holy book.

The defense secretary said he had been notified by the U.S. commander of the NATO-led force in Afghanistan, General John Allen, "of the deeply unfortunate incident involving the inappropriate treatment of religious materials, including the Koran, at Bagram Airbase."

"He and I apologize to the Afghan people and disapprove of such conduct in the strongest possible terms," Panetta said, adding that the actions did not represent the views of the U.S. military.

"I support General Allen's swift and decisive action to investigate this matter jointly with the Afghan government," he said.

"I will carefully review the final results of the investigation to ensure that we take all steps necessary and appropriate so that this never happens again."