US President Barack Obama was Thursday forced to apologise over the burning of Korans at a US airbase in Afghanistan, where three days of protests have killed 14 people, including two American soldiers.
Violent anti-US protests have seen furious Afghans attack French, Norwegian and US bases, shouting "death to America" after the Taliban exhorted their countrymen to kill foreign troops to avenge the incident at a US-run base.
Afghanistan is a deeply religious country where slights against Islam have frequently provoked violent protests, and many Afghans are incensed at the discovery of charred Korans at the US-run Bagram airbase north of Kabul.
In a letter of apology to Afghan President Hamid Karzai, Obama expressed "deep regret" over the incident that he said was unintentional and pledged that those responsible would be held accountable, Kabul said.
"I extend to you and the Afghan people my sincere apologies," Obama wrote in the letter presented to Karzai by US ambassador Ryan Crocker.
"The error was inadvertent; I assure you that we will take the appropriate steps to avoid any recurrence, to include holding accountable those responsible."
Karzai said a US officer was responsible -- "out of ignorance" -- for the Koran burning at Bagram airbase north of Kabul and the US government had admitted the mistake, his office said.
The two Americans were shot dead when an Afghan soldier turned his weapon on them at their base in Khogyani in eastern Nangarhar province, district governor Mohammad Hassan told AFP.
"As the protesters approached the American base here an ANA (Afghan) soldier in the base opened fire on American soldiers, killing two soldiers," he said.
NATO's US-led International Security Assistance Force (ISAF) said two of its service members were killed in the east by "an individual wearing an Afghan National Army uniform" who turned his weapon against the troops.
ISAF did not identify the nationalities of the victims.
One Norwegian soldier was lightly injured when a hand grenade was thrown into a NATO camp in Maimana, capital of northern Faryab province, during a demonstration, the Norwegian army said.
At least three Afghans were also killed by gunfire at demonstrations in the south and east of the country, bringing the total death toll among protesters to 12 since Wednesday.
The violence came after the Taliban urged Afghans to kill foreign troops to avenge the Koran burning, although the insurgents stopped short of cutting off tentative peace contacts with American officials in Qatar.
In Mihtarlam, the capital of Laghman province east of Kabul, thousands besieged the base of a US-led military-civilian provincial reconstruction team (PRT), throwing rocks and climbing up the outer walls, police said.
"People had come from all over Laghman. They attacked the PRT, they climbed up the walls, they set fire to something there, I think a container," police official Khalilul Rahman Niazi told AFP.
Niazi said he believed two people were wounded by gunfire from the base as they stormed the walls and hurled rocks under a pall of thick black smoke.
About 2,000 protesters also tried to march on the French base in Kapisa, east of Kabul, but were pushed back by Afghan security forces, regional police chief General Abdul Hameed Erken told AFP.
"Two protestors were slightly wounded after security forces opened fire on them," he said.
Karzai had called for calm pending a full investigation, and ordered his own security forces to avoid violence and protect people's lives and property.
But the Taliban, leading a 10-year insurgency against Karzai's government, on Thursday sought to exploit the anti-American sentiment.
"You should bring the invading forces' military bases under your brave attack, their military convoys, kill them, capture them, beat them and teach them a lesson that they will never again dare to insult the Holy Koran," it said in a statement.
The Islamist movement was toppled in the 2001 US-led invasion. NATO has some 130,000 troops, mainly Americans, supporting the Karzai government.
US officials speaking on condition of anonymity told AFP the military removed Korans from a prison at Bagram because inmates were suspected of using the holy book to pass messages to each other.