The Islamic State group is relying more heavily on child soldiers as growing numbers of fighters desert the jihadist organisation, the United States has said.
That may indicate the group's leaders are "struggling with their ability to recruit and retain manpower," State Department spokesman John Kirby told reporters during a daily briefing on Monday.
He would not confirm reports that Kurdish Peshmerga forces in northern Iraq had captured a US citizen who defected from IS.
"We are working closely with the Iraqi and Kurdish authorities to try to get more information to confirm the veracity of these reports," Kirby said.
However, "more and more" defectors are leaving IS's ranks, prompting the group to rely more heavily on child soldiers, the spokesman added.
"Originally, they would rely on children for intelligence streams, getting information... and then using them to conduct suicide attacks, which they still do," Kirby said.
"Now we get more reports about them using children in actual engagements side by side with adult fighters," he added.
"All those are good indications they are struggling with their ability to recruit and retain manpower."
However, the United States still considers the threat from IS "very seriously," Kirby said. "They are still very lethal."
US forces have led a military coalition of 60 countries fighting IS for nearly two years.
The jihadist group controls vast swathes of territory in Iraq and Syria.
Russia has announced it would begin withdrawing its forces from Syria, saying its bombing campaign had helped "radically change the situation in the fight against terrorism."