KABUL, Afghanistan — A gunman killed two American advisers Saturday inside a heavily guarded government compound in Kabul, officials said, as protests against the burning of copies of the Muslim holy book roiled the country for a fifth day.

U.S. officials said the assailant remained at large as neither an apology from President Barack Obama nor gunfire from Afghan police could quench public outrage over what NATO insisted was an inadvertent desecration of the Quran.

Two Afghan officials said the ministry shooting did not involve any Afghans. They spoke anonymously to discuss a NATO incident. One of the officials noted that the shooting occurred inside a secure room at the ministry that Afghan staff do not have access to.

NATO confirmed that two service members were killed, but spokesman Lt. Col Jimmie Cummings said “initial reports say it was not a Western shooter.” He declined to provide further information.

A U.S. official in Washington confirmed that the two killed were American.

“The assailant is unknown, and an aggressive search is under way to determine who is responsible,” Pentagon press secretary George Little said.