KABUL, Afghanistan (AP) -- NATO's top military commander said Friday that an investigation into this week's Quran burnings at a U.S. base in Afghanistan is pushing ahead and called on Afghans to be patient and exercise restraint.
U.S. Gen. John Allen's call came as Afghan security forces were put on high alert to deal with expected demonstrations around the country after Friday prayers.
"Working together with the Afghan leadership is the only way for us to correct this major error and ensure that it never happens again," Allen said in a statement.
He called on everyone around Afghanistan "to exercise patience and restraint as we continue to gather the facts."
The U.S.-led military coalition says the Muslim holy books were sent by mistake on Feb. 19 to a garbage burn pit at Bagram Air Field, near the capital, and that the case is under investigation. The explanation and multiple apologies from U.S. officials have yet to calm outrage over the incident, which has also heightened tension between international troops and their Afghan partners.
The country has been wracked by three days of violence that has claimed 13 lives, including two U.S. soldiers over the burnings of the Muslim holy book.
The level and intensity of any demonstrations Friday will also test whether Thursday's apology by President Barack Obama to the Afghan people over the incident will have any impact.
Afghans have so far ignored an appeal by President Hamid Karzai and other officials for calm.
On Thursday, thousands of protesters, some shouting "Long live Islam!" and "Death to America!" staged demonstrations across Afghanistan for a third day. Protesters climbed the walls of a U.S. base in the east of the country, threw stones inside and adorned an outside wall with the Taliban's trademark white flag.
The protests sparked clashes with Afghan security forces that left at least five demonstrators dead. A Norwegian soldier was wounded by a hand grenade hurled into a coalition compound.
On Wednesday, six people died in similar protests in Kabul and three other provinces.
The unrest started Tuesday, when Afghan workers at the sprawling American base north of Kabul noticed that Qurans and other Islamic texts were in the trash that coalition troops dumped into a pit where garbage is burned. Some Afghan workers burned their fingers as they tried to salvage some of the books. Afghan government officials said initial reports indicated four Qurans were burned.
The materials had been taken from a library at Parwan Detention Facility, which adjoins the base, because they contained extremist messages or inscriptions. Writing inside a Quran is forbidden in the Islamic faith, and it is unclear whether the handwritten messages were found in the holy book or other reading materials.
A military official said it appeared that detainees at the prison were exchanging messages by making notations in the texts.
A delegation of Afghan religious leaders, lawmakers and government representatives visited Bagram as part of the investigation. They issued a statement late Thursday calling for an end to protests and accused insurgents of infiltrating the gatherings to foment violence. They said they expected those responsible for the Quran burning to be prosecuted through the U.S. military court system.
Afghan authorities said demonstrations were staged inside the capital and in seven of Afghanistan's 34 provinces.
The deadliest was held outside an American base in the Khogyani district of Nangarhar province in eastern Afghanistan. Two protesters were killed by Afghan police and an Afghan soldier turned his gun on American troops, killing two.
Associated Press writer Deb Riechmann in Kabul contributed to this report.