President Barack Obama has issued a letter of apology to Afghan President Hamid Karzai in response to the burning of copies of Koran at NATO's Bagram airbase, which provoked violent protests beginning Wednesday that have resulted in several deaths, including two coalition soldiers, believed to be Americans, shot dead by a man in an Afghan Army uniform.

I wish to express my deep regret for the reported incident, President Obama said in the letter to President Karzai. I extend to you and the Afghan people my sincere apologies. The error was inadvertent. I assure you that we will take the appropriate steps to avoid any recurrence, to include holding accountable those responsible.

In an interview with PBS News Hour, Associated Press correspondent Heidi Vogt, citing anonymous NATO officials, reported that copies of the Koran had been confiscated at a NATO detention facility, and were flagged for incineration due to writing in the margins that contained extremist messages promoting a very radical form of Islam and correspondences between detainees.

As leader of NATO operations in Afghanistan, the U.S. has drawn the majority of the blow-back for the Koran burnings, which were only affirmed by a preliminary investigation into the incident, which showed that American soldiers had burned four copies of the Holy Koran, according to a statement Wednesday by President Karzai.

Karzai has also condemned the violence saying, Protests are the right of people but I ask my countrymen to avoid violence.

While the President's apology and the pending results of a full investigation aim to quell the recent violence, the Taliban, which maintains power and influence in mountainous regions of Afghanistan's southern border with Pakistan, have called on Muslims to attack NATO military bases and coalition troops, according to CNN.

We should attack their military bases, their military convoys, we should kill their soldiers, arrest their invading soldiers, beat them up and give a kind of lesson to them that they never dare to insult the holy Koran, said an e-mail message sent out by the Taliban, CNN reported.

Some members of the Afghani Parliament have also continued to express outrage over the burnings of the Islamic holy book.

Americans are invaders, and jihad against Americans is an obligation, said Abdul Sattar Khawasi, a member of Parliament from the northeastern province of Parwan, according to the New York Times.

NATO officials have expressed concern that radical Islamic leaders could incite an escalation of violence during Friday prayers when many Muslims are gathered at mosques, the Times reported.