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Baghdad • 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 their way 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 wasn't 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. A U.S. statement confirmed that two soldiers were killed and a third was wounded by small-arms fire from 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.

The Americans weren't identified pending notification of next of kin, and the statement provided few other details. The U.S. troops were from the 4th Brigade, 1st Cavalry Division, based at Ft. Hood, Texas.

Additionally, another American soldier was killed Saturday during an unrelated military operation in central Iraq, making it one of the deadliest days for U.S. forces in the country in months. A U.S. military statement offered no details about that death.

The Mosul attack underscores the threats that U.S. forces continue to face in Iraq, even though most of the estimated 47,000 troops no longer go on regular combat missions. The vast majority of American troops left — down from nearly 170,000 in 2007 — are all but confined to bases, where they help train Iraqi police, soldiers and pilots how to protect the country from threats such as insurgents and invasions.