The Islamic State group is relying more heavily on child soldiers as growing numbers of fighters desert the jihadist organization, the United States said Monday.

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.

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."

American IS member surrenders to Iraq Kurds: general
Arbil, Iraq (AFP) March 14, 2016 - A member of the Islamic State jihadist group with US citizenship turned himself in to the Kurdish peshmerga forces in northern Iraq on Monday, a senior military officer said.

"This person's name is Mohammed Jamal Amin, he holds US citizenship, has a Palestinian father and an Iraqi mother," Major General Hashem Siti, commander of the peshmerga's 8th brigade, told AFP.

He said the IS member was carrying $4,000 in cash and surrendered in a village near Sinjar, a city that Kurdish forces retook from IS last year.

It was not immediately clear how long the suspect had been a member of IS and what his role was.

US officials in Washington could not immediately confirm the account provided by the peshmerga.

"We are aware of the reports that a US citizen allegedly fighting for ISIL (IS) has been captured by peshmerga forces in northern Iraq," Pentagon spokesman Jeff Davis told reporters.

"We are in touch with Iraqi and Kurdish authorities to determine the veracity of the report," he said.

The United States is not one of the main sources of foreign fighters to IS, with fewer than 300 US nationals believed to have joined the jihadist group in Iraq and Syria.