Six Defense Department officials, all speaking on condition of anonymity because they were not authorized to describe how Islamic State targets are selected and attacked, described a sliding scale of probable civilian casualties based on the value of the target and the location. For example, a strike with the potential to wound or kill several civilians would be permitted if it prevented ISIL fighters from causing greater harm.

Before the change, there were some limited cases in which civilian casualties were allowed, the officials said. Now, however, there are several targeting areas in which the probability of 10 civilian casualties are permitted. Those areas shift depending on the time, location of the targets and the value of destroying them, the officials said.

The riskiest missions require White House approval, said one official, who is closely involved with current targeting plans.