KABUL—Afghanistan was bracing for intensifying protests, even as outrage over the burning of Qurans at a U.S. base led to the first American casualties and President Barack Obama vowed to hold coalition officials accountable for the "inadvertent" sacrilege.

The three days of protests against Monday's burning of Islam's holiest book at the Bagram Airfield already have resulted in the deaths of at least 10 Afghans. Demonstrations are expected to escalate Friday, when clerics at mosques around the country are expected to dedicate...