An Afghan soldier joined protests on Thursday against the burning of copies of the Muslim holy book at a NATO base and shot dead two foreign troops, western military sources said.
The killings came hours after the Taliban urged Afghans to target foreign military bases and kill Westerners in retaliation for the burning of the Korans at Bagram airfield on Tuesday.
Eleven people have died in demonstrations across the country since then and 17 people have been wounded. Thousands of protesters have taken to the streets of several cities, chanting Death to America! and smashing cars, buildings and shops.
In a demonstration in eastern Nangarhar province on Thursday, an Afghan soldier turned his gun on NATO soldiers, local officials and western military sources said. A provincial spokesman said the soldier then escaped.
NATO confirmed a man in Afghan army uniform killed two of its troops in the east, but declined to say if the shooting was connected to the protests.
The Koran burnings at the vast Bagram base north of Kabul, which the United States has said were unintentional, could make it even more difficult for U.S.-led NATO forces to win the hearts and minds of Afghans and bring the Taliban to the negotiating table ahead of the withdrawal of foreign combat troops by the end of 2014.
Muslims consider the Koran the literal word of God and treat each book with deep reverence. Desecration is considered one of the worst forms of blasphemy.
A small protest of around 500 people also turned violent in the capital Kabul, with gunfire crackling above the city as police and plain-clothed intelligence officers charged demonstrators wearing bandanas and hurling rocks and sticks, firing low above their heads and sending them fleeing.
A wounded youth lay on the frozen asphalt on a road, blood pouring from his side. Crouched over and cradling him, a relative appealed to the government to not hurt its own people.
Ministry of the Interior! Don't you see we are fighting NATO? said the man, who did not give his name.
Masked men sped by on a motorcycle blasting a battle song played by the Taliban insurgency, while police in machine gun-mounted pick-up trucks picked up the wounded.
Our brave people must target the military bases of the invaders, their military convoys and their invader forces, read an e-mailed Taliban statement released by the insurgency's spokesman Zabihullah Mujahid.
They have to kill them (Westerners), beat them and capture them to give them a lesson to never dare desecrate the holy Koran again.
Most Westerners were already confined to their heavily fortified compounds, including within the sprawling U.S. embassy complex and nearby embassies in central Kabul.
Around 400 protesters hurled rocks and set fire to cars at a Norwegian-led military base in Faryab province on the Turkmen border, which is centre for around 500 soldiers and civilians from Norway, Latvia, Macedonia, Iceland and the United States.
Norway's ambassador to Kabul, Tore Hattrem, told Reuters no one was hurt and there was minimal damage.
A small number of protesters in the eastern Kapisa province took aim at the French military base there, though police deterred them successfully, its police chief Abdul Hamid said.
The venting of fury could complicate efforts by U.S. and NATO forces to reach agreement with the Afghan government on a strategic pact that would allow a sharply reduced number of Western troops to stay in the country, well beyond their combat exit deadline, to oversee Afghan forces.
Underscoring these concerns, hundreds of students in Jalalabad rejected any strategic pact with the United States, saying they would take up jihad if one were sealed.
The U.S. government and the American commander of NATO-led forces in Afghanistan apologised for the burnings after Afghan labourers found charred copies of the Koran while collecting rubbish at Bagram.
A report into the incident by NATO investigators and a team of senior Afghan clerics was to be handed to Karzai as soon as Thursday.
Martine van Bijlert, from the respected Afghanistan Analysts Network (AAN), said the demonstrations were a combination of religious outrage, pent-up frustration over economic and security conditions, and groups wanting to stir trouble.
There have been different kinds of outrage. One is the bewilderment felt by many Afghans, and foreigners, that after ten years of efforts in Afghanistan there was apparently still no understanding of how inflammatory mistakes like that are made, van Bijlert said on the AAN website.
Second, there is the pent-up anger and frustration with the international military, but also with life in general.
(Additional reporting by Mohammad Hamid in KUNDUZ, Rafiq Shirzad in JALALABAD, Akram Walizada and Omar Sobhani in KABUL, Elyas Wahdat and Obaidullah in LOGAR,; Writing by Amie Ferris-Rotman and Rob Taylor; Editing by Nick Macfie)