Kabul, Afghanistan (CNN) -- Religious materials -- including Qurans that were burned at Bagram Airfield in Afghanistan, sparking Afghan protests -- were removed from the library of a detainee center "because of extremist inscriptions" on them, a military official said Tuesday.
There was "an appearance that these documents were being used to facilitate extremist communications," a military official said.
"Additionally, some of the documents were extremist in and of themselves, apparently originating from outside of Afghanistan," the official said.
The official said the material was burned, but authorities are attempting to determine how much.
The uproar prompted Gen. John Allen, commander of NATO's International Security Assistance Force, to issue a directive "that all coalition forces in Afghanistan will complete training in the proper handling of religious materials no later than March 3," the NATO-led force said..
The training will include "the identification of religious materials, their significance, correct handling and storage," according to the statement from coalition forces.
Earlier Tuesday, Allen said the materials were gathered for disposal from the airfield's Parwan detention facility and inadvertently given to troops for burning.
"This was not a decision that was made because they were religious materials," he said. "It was not a decision that was made with respect to the faith of Islam. It was a mistake. It was an error. The moment we found out about it, we immediately stopped and we intervened."
Hundreds of demonstrators gathered outside the airfield Tuesday, furious over reports of the burning.
Lt. Lauren Rago, a spokeswoman for the coalition forces, told CNN in an e-mail earlier Tuesday that an investigation was under way to determine whether any material was burned before the mistake was discovered and, if so, how much.
"We are deeply concerned about the report of Qurans or religious materials being damaged, and will get to the bottom of what actually happened," she said.
But a coalition official acknowledged the materials were improperly burned.
Allen said he immediately launched an investigation.
"Something like this just cannot happen again," Allen said.
Muslims regard the Quran as the absolute word of God. It is so highly revered that many Muslims will not pick up the holy book without ablution, a ritual washing of the hands.
Desecrating the book, such as burning it, is therefore seen as an unforgivable affront -- as an act of intolerance and bigotry.
Authorities are looking into the reasons why the material was gathered and why the decision was made "to dispose of them in this manner," Allen said.
Some troops have been questioned, but there have been no arrests or detentions, according to a coalition official. Afghan officials have been invited to join the investigation.
Allen said he has offered "sincere apologies" to Afghan officials, including President Hamid Karzai. Allen also offered apologies to the Afghan government and "the noble people of Afghanistan."
"This was unintentional," he said. "There was no intention by any member of ISAF to defame the faith of Islam or to desecrate the precious religious materials of this faith."
"The materials recovered will be properly handled by appropriate religious authorities," Allen said earlier.
Local citizens who work at the base discovered the material that had been put into a burn pit by NATO personnel and alerted officials.
Soon after, demonstrators massed outside the base, chanting "Death to America! Death to the Afghan government! Long live Islam!"
As a NATO helicopter circled overhead and black smoke billowed from the ground, men shook their fists in anger. Several men launched slingshots in the direction of the base.
"The desecration of religious articles is not in keeping with the standards of American tolerance, human rights practices and freedom of religion," State Department spokeswoman Victoria Nuland said Tuesday.
"The concern is that some around the world will think that this represents an affirmative statement by the United States, and it does not represent our values or our view of how the Quran ought to be treated," she said.
Allen said, "This is not who we are. These are very, very isolated incidents. We've been here a long time. We've been shoulder to shoulder with the Afghans for a long time. We've been dying alongside the Afghans for a long time because we believe in them. We believe in their country. We want to have every opportunity to give them a bright future."
But "these kinds of incidents, when they do occur, we will move quickly to correct them," he said. "We will move quickly to hold people accountable."
Photographs surfaced purporting to show the damaged Qurans. A photographer for Agence France-Presse said Afghans who work inside the airfield told him they obtained the Qurans there.
But the U.S. military said that was unlikely.
"When it became known that it was religious materials that were brought to the incinerator, the materials were secured by the military authorities in consultation with the local Afghan religious authorities, so it would be highly unlikely that the demonstrators would have any of the material from this incident," said Col. Gary Kolb, a spokesman for the NATO-led force.
The U.S. Embassy in Kabul warned on its Twitter feed that protests were possible throughout Afghanistan in the coming days and noted, "Past demonstrations in Afghanistan have escalated into violent attacks on Western targets of opportunity."
Last year, when controversial Florida pastor Terry Jones presided over what he called a trial of the Quran and burned a copy, Afghans took to the streets by the thousands. In the northern Afghan city of Mazar-e Sharif, demonstrators stormed a U.N. office and killed 12 people. In Kandahar, three people were killed in one demonstration, and nine in another when police and stone-throwing demonstrators clashed.
American officials vociferously condemned the pastor's act.
"It was intolerant and it was extremely disrespectful and again, we condemn it in the strongest manner possible," said Gen. David Petraeus, who headed the U.S.-led international forces in Afghanistan at the time.
In 2010, Afghans protested outside the Forward Operating Base Mirwais in response to an alleged Quran burning inside the base. But coalition forces said the suspected burning was a routine burn-pit session in which military documents are destroyed.
CNN's Barbara Starr and Larry Shaughnessy in Washington, Masoud Popalzai in Kabul and Sarah Jones and Ashley Hayes in Atlanta contributed to this report.