Colonel Pat Ryder of the US military's Central Command said that the number of boy soldiers -- some as young as 10 -- had been growing steadily, and that some are even being used to put prisoners to death.
"We've seen this trend continuing to increase for a while now, where they are having to, through forced conscription, recruit children as young as 10 to go fight," Ryder said, calling the practice "obviously disgusting and illegal across the board."
"This again is an indication that they are scraping to replace the losses that they've experienced on the battlefield," he said.
The US military issues no official counts of enemy dead -- it was criticized during the Vietnam War for using "body counts" as a metric of progress -- as it leads the multinational coalition conducting air strikes against IS fighters in Iraq and Syria. But Pentagon estimates suggest the number of slain jihadists is between 23,000 and 33,000.
Ryder's comments came the day after the IS group released a horrific video showing six of its child soldiers killing Syrian security forces captured by the jihadists.
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When the six boys are chosen by an instructor to "send a message" to IS's opponents, five of them shoot and kill Syrian captives, while the sixth slits a prisoner's throat.
The slick video, titled "To the Sons of Jews," also shows dozens of boys studying religious texts and learning hand-to-hand combat. Through a program it calls "Cubs of the Caliphate," IS provides intense military and religious training to children in the area it controls.
Although child soldiers have been used before to guard checkpoints or gather intelligence, IS has begun increasingly using them to execute prisoners.
"It's just sickening," Ryder said.
Asked if the coalition would knowingly bomb child soldiers or if their presence on the battlefield alters the military's rules on when deadly force can be used, Ryder said the coalition tries to avoid killing "any innocent person on the battlefield."
But "the reality is, if an ISIL fighter is armed and attacking, then that ISIL fighter is a legitimate military target," he said, using an alternative acronym for the IS group, which is also known as ISIS and Daesh.
He noted that the "overwhelming majority" of IS fighters are "military-aged males."