NATO officials say an individual wearing an Afghan army uniform shot and killed two coalition soldiers in eastern Afghanistan during one of the many protests that have erupted across the country over the burning of Qurans at a NATO facility.
The deadly incident occurred Thursday outside a coalition military base in Nangarhar province.
Earlier Thursday, the Taliban issued a statement calling on Afghans to launch attacks on foreign targets in retaliation for the desecration of the Muslim holy book.
But in a joint statement, the Afghan delegations assigned to probe the incident appealed to the Afghan people to "exercise self-restraint and extra vigilance" and avoid resorting to protests that may allow "the enemy to take advantage of the situation."
Since the demonstrations erupted Tuesday, clashes between Afghan security forces and protesters have left at least 13 people dead.
On Thursday, hundreds of protesters attacked a U.S.-run base in eastern Laghman province, where they stormed the walls and threw rocks. In northern Baghlan province, officials said gunfire killed one protester and wounded at least two police officers.
Meanwhile, Afghan President Hamid Karzai's office said it has received a letter from U.S. President Barack Obama formally apologizing for the incident "in which religious materials were unintentionally mishandled."
In the letter, Obama expresses his "deep regret" and offers his "sincere apologies." He writes that "the error was inadvertent" and that officials will take the "appropriate steps" to avoid any recurrence and hold those accountable responsible.
On Wednesday, President Karzai appealed for calm, saying citizens have the right to protest, but should not resort to violence.
Nazif Shahrani, an Afghan native who is a South Asia analyst at Indiana University, told VOA the protests are about more than this one incident.
"What people seem to perceive is that the U.S. has essentially not kept its promises and that all the promises have been false," said Shahrani. "And these are really symptoms of that anger and that disappointment toward the United States. And it's hoped that the United States government looks at this issue more broadly than just apologizing for what has happened in Bagram."
U.S. Deputy Defense Secretary Ashton Carter met Wednesday in Kabul with Afghan leaders, including President Karzai, to again apologize for the incident.
The commander of the international coalition, U.S. General John Allen, had issued an apology Tuesday, saying the improper disposal of Islamic religious texts was "not intentional in any way." He has ordered an investigation.
Protesters said Wednesday that they want the Americans out of their country and that words alone cannot change the disrespect that Muslims have suffered.
Media reports quote unnamed Western officials with knowledge of the incident as saying it appeared that the copies of the Quran in question and other Islamic readings in the library at Bagram were being used to fuel extremism, and that detainees were writing on the documents to exchange extremist messages.
Afghan protests against the destruction of the Muslim holy book have turned deadly in recent years. In April 2011, about 20 people were killed during several days of protests across Afghanistan after little-known U.S. pastor Terry Jones burned a Quran at his small Florida church.
Some information for this report was provided by AP, AFP and Reuters.