CBS/AP

Riots have died down in Afghanistan more than a week after the revelation that copies of the Quran were burned at an American base, but the death toll among Americans continues to rise.

Two U.S. troops were killed Thursday by assailants believed to be Afghan soldiers, bringing the number of troops killed to six. At least 15 have been wounded. The death toll among Afghans after days of protesting is much higher.

The latest victims were shot exactly one week after the first U.S. soldiers were killed; Army Sgt. Joshua Born, 25, and Cpl. Timothy Conrad, 22, were shot during a protest on Feb. 23.

A U.S. lieutenant colonel and major were found in dead two days later, both shot in the back of the head. The bodies were discovered in an office inside the heavily-fortified Interior Ministry, leading Afghan officials to believe the killer was an employee of the ministry. Afghan police are still searching for the suspect.

The U.S. maintains that the Quran incident, though a setback in their mission, will not undo progress made in the last 10 years.

"A decade's worth of relationships doesn't go away in a single week," U.S. Ambassador to Afghanistan, Ryan Crocker, told CBS News.