Violent protests during the past three days over the burning of Korans at a U.S. airbase in Afghanistan have left 15 people dead, including two American soldiers.

Military sources say an Afghan soldier taking part in the protest shot dead two American soldiers at their base in Khogyani in eastern Nangarhar province on Thursday. Nangarhar’s district governor, Mohammad Hassan, said two protesters also died in a gunfight that erupted after the Americans were killed.

The White House says U.S. President Barack Obama issued an official apology over the burning of the Korans at the U.S. airbase after protests grew violent.