President Barack Obama apologized to Afghans on Thursday for the burning of Qurans at a U.S. military base, trying to assuage rising anti-American sentiment as an Afghan soldier gunned down two American troops during another day of angry protests.

The U.S.-led military coalition says the Muslim holy books were sent by mistake to a garbage burn pit at Bagram Air Field and 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.

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, threw stones inside and adorned an outside wall with the Taliban’s trademark white flag.

At other sites, demonstrators burned tires or American flags. Afghan police and international troops fired guns in the air to disperse the crowds.