President Hamid Karzai on Thursday blamed a U.S. soldier for ignorantly burning copies of the Koran at a NATO base, an accusation that could trigger more protests across Afghanistan.

Demonstrations against the burning of copies of Islam's most holy book drew thousands of angry Afghans to the streets, chanting Death to America! for the third consecutive day in violence that has killed 11 people including two foreign troops and wounded many more.

President Barack Obama sent a letter to Karzai apologizing for the burning of the Korans at the sprawling Bagram airbase. Obama told Karzai the incident was not intentional.

The letter, which the White House said was a follow-up to a phone call earlier this week between the two leaders to discuss a long-term partnership between Washington and Kabul, was delivered to Karzai by U.S. Ambassador Ryan Crocker.

The Koran burnings could make it even harder 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.

Karzai said the American officer had acted out of ignorance and with poor understanding of the Koran's importance, a presidential statement said.

Karzai's office said an investigation into the Koran burning would most likely be concluded later Thursday.

Protests could swell Friday if mosque preachers focus on the incident, and stir up anti-American feelings, as they have done after similar incidents in the past on the Muslim holy day.

The Taliban also urged security forces to turn their guns on the foreign infidel invaders, it said on its site shahamat-english.com.

An Afghan soldier shot dead two NATO soldiers during a protest at a base in Nangarhar province, local officials and western military sources said.

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.

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 and there could be further trouble Friday when congregational prayers are held.

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.

FOREIGN BASES ATTACKED

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.

Twelve protesters were wounded in the attack, the head of the regional hospital Abdul Alim said, but 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 anger 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.

Martine van Bijlert, from the 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 Michael Georgy and Ed Lane)