Kabul: Protests erupted outside the main US military base in Afghanistan and in Kabul on Tuesday over allegations that foreign troops had set fire to copies of the Koran, police said.

Thousands of protesters besieged Bagram airbase, north of Kabul, throwing petrol bombs which set fire to a gate.

The US commander in Afghanistan, General John Allen, said he had ordered an investigation into a report that troops at Bagram "improperly disposed of a large number of Islamic religious materials which included Korans".

 "I offer my sincere apologies for any offence this may have caused, to the president of Afghanistan, the government of the Islamic Republic of Afghanistan, and most importantly, to the noble people of Afghanistan," he said.

A local police official said more than 2,000 people were demonstrating outside the sprawling US-run base.

"They are demonstrating over the burning of copies of the Koran inside the base," the official said.

Sidiq Siddiqi, an interior ministry spokesman, confirmed the demonstration and said reinforcements were sent to the area, about 60 kilometres (40 miles) north of the capital to prevent possible violence.