Two U.S. troops were killed Saturday by an Iraqi soldier who apparently smuggled real bullets into a training exercise and opened fire, raising fresh concerns about insurgents worming into the nation's security forces as the Americans prepare to leave by the year's end.

A U.S. military official said the shooter was immediately killed by American soldiers who were running the morning drill at a training center on a U.S. base in the northern city of Mosul. The U.S. official said the exercise was not meant to involve live ammunition, and an Iraqi army officer said the shooting appeared to have been planned.

Both officials spoke on condition of anonymity because they were not authorized to release the information. A U.S. statement confirmed that two soldiers were killed and a third was wounded by small-arms fire by what the military described as "an individual wearing an Iraqi army uniform."

"This incident occurred during a training event being conducted by U.S. forces as part of their advise and assist mission with Iraqi security forces," the U.S. military said in a statement.