A video on a boy's phone shows him firing a heavy machine gun mounted on a tripod through a hole in a crumbling building, his slender body shaking from the kickback.

"We were kind of nervous, not because we are cowards, because it was our first fight and we were still young," said the youth, a former teen soccer star from Baghdad.

He was 15 when an Iraqi Shiite militia first sent him to Iran for training by the Revolutionary Guard in hills outside of Tehran in March. He spent his 16th birthday four months ago in Syria fighting on the front line near Damascus.