US Defence Secretary Leon Panetta on Tuesday apologized to the Afghan people following reports that coalition troops "improperly disposed" of Islamic religious materials, including copies of the Koran, at the Bagram air base, dpa reported.
Panetta said in a written statement that he and General John Allen, the commander of the coalition forces, "disapprove of such conduct in the strongest possible terms."
"These actions do not represent the views of the United States military. We honour and respect the religious practices of the Afghan people, without exception," Panetta said.
Thousands of Afghans took to the streets in northern Afghanistan Tuesday to protest at the "Koran burning" by NATO-led troops, officials said.
Allen said some of his soldiers had "improperly disposed" of Islamic religious materials late Monday at the Bagram air base, north of Kabul.
Panetta said the incident would be investigated jointly with the Afghan government, and that he would carefully review the final results "to ensure that we take all steps necessary ... so that this never happens again."
Earlier Tuesday, Roshana Khalid, a spokeswoman for Parwan's provincial government, told dpa: "Afghan labourers at the ... air base brought copies of the Koran burnt by the coalition troops out of the base this morning."
About 3,000 people gathered outside the base to protest the burning, she said.
Zia ul Rahman, the deputy police chief of Parwan province, said: "The protest turned violent but we arrived at the site right on time and took the situation under control.
"Rubber bullets were fired by the Americans in which 13 people were slightly injured."
Colonel Jimmie Cummings, an ISAF official, said, "Non-lethal munitions were used early to assist with the dispersing of the crowd outside of Bagram."
Protesters, mainly from nearby villages, shouted "death to Americans," demonstrator Sami Ullah told dpa.
Allen said, "The materials recovered will be properly handled by appropriate religious authorities."
He added: "I assure you ... I promise ... this was not intentional in any way." Allen also offered a personal apology to Afghan President Hamid Karzai and the Afghan government.