"They resulted to tactics that we’ve never seen before. I’ve been in the military for 31 years and I’ve never seen an adversary like this — I’ve never read about an adversary like this that exploits the population the way that they have," Martin told reporters.
Martin explained that Daesh fighters used Iraqi civilians as sex slaves, slave labor and in other ways.
"They’ve used them just about any way they could," he added.
US-led coalition airstrikes have also been blamed for civilian deaths. Last week, the UN Office for the Coordination of Humanitarian Affairs said a US strike on May 31 killed between 50 and 80 civilians. The same UN report found that Daesh militants killed many more, with the terror group's snipers, bombs and shelling killing around 230 people in the same period.