The US military removed Korans from a US-run prison in Afghanistan because inmates were suspected of using the sacred book to pass messages to each other, American officials said Tuesday.

The Korans and other religious material were later burned at the Bagram airfield, causing outrage in Afghanistan and triggering a protest Monday with petrol bombs hurled outside the sprawling US base, which includes a large prison.

"The material was removed because there was a concern that the detainees were communicating with each other," one US official, who spoke on condition of anonymity, told AFP.

But the Korans and other texts should have been removed from the prison in a more appropriate manner, two US officials said.

The account raised fresh questions about the incident, the latest public relations headache for the NATO-led mission after a damaging video of US Marines urinating on Taliban corpses and after an American pastor torched a Koran in April.

The top commander of US and NATO troops in Afghanistan, General John Allen, quickly issued an abject apology and insisted the episode was an accident.

"I assure you -- I promise you -- this was NOT intentional in any way," he said in the statement.

The US-run Parwan prison at Bagram houses roughly 3,000 detainees captured by coalition forces in their fight with Islamist insurgents.