U.S. President Barack Obama apologized to Afghan President Hamid Karzai Thursday for NATO troops\' burning of Korans, which has led to violent protests. An Afghan soldier, apparently angry over the burning of Korans at a U.S. air base, fatally shot two U.S. troops and wounded four others, Afghan officials said. National Security Council spokesman Tommy Vietor and Karzai\'s office said Obama had called the burning of the Muslim sacred text at Bagram Air Base this week an \"error,\" CNN reported. \"We will take the appropriate steps to avoid any recurrence, including holding accountable those responsible,\" Obama said in the letter, delivered by U.S. Ambassador to Afghanistan Ryan Crocker. CNN said at least five people were killed in demonstrations Wednesday. It remained unclear how many Korans were involved, a military official told CNN Thursday. The official said American troops at the base would have been unable to read the texts, which could have contributed to the error. Another military official said earlier the materials had been removed from a detainee center\'s library because they carried \"extremist inscriptions\" on them and there was \"an appearance that these documents were being used to facilitate extremist communications.\" The International Security Assistance Force said in a statement the two military personnel were killed in eastern Afghanistan Thursday by \"an individual wearing an Afghan National Army uniform\" but didn\'t identify the troops\' nationality. CBS News said an Afghan official said the dead and wounded in the attack in the eastern province of Ningarhar were American. The official said the shooting seemed to be motivated by the burning of Korans at the Bagram Air Base north of Kabul, but did not elaborate. Haji Mohammad Hassan, chief of Khugyani district in Ningarhar province, told CNN, \"We don\'t know who started the shooting first and what kind of guns were used, but we have started our investigation to find out the details of the incident.\" Violent anti-American protests have erupted across Afghanistan since the U.S. military Tuesday apologized for what it said was the accidental \"improper disposal\" at Bagram of religious materials, including copies of the Muslim holy book. The United States is working with the Afghan government to investigate the incident. CBS said violent protests took place again on Thursday at U.S. and NATO military facilities across Afghanistan, including one demonstration in which police apparently shot and killed two protesters. CNN also reported the Taliban in Afghanistan called on Muslims Thursday to attack NATO facilities and convoys and kill military personnel. In an e-mail message, the Taliban accused \"the invading infidel authorities\" of trying to calm the situation with two \"so-called shows of apology, but in reality they let their inhuman soldiers insult our holy book.\" The e-mail urged Afghans to seek revenge \"until the doers of such inhumane actions are prosecuted and punished.\" \"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,\" the message said.