WASHINGTON -- The U.S. military says it shot down what it called an Iranian-made, armed drone in southern Syria.

CBS News correspondent David Martin reports the drone came too close to one of the outposts of U.S.-backed fighters outside of their main base in Tanf. U.S. forces have been training and advising local Syrian Arabs for the fight against Islamic State of Iraq and Syria (ISIS) at the base.

A defense official says the drone was considered a threat. It was shot down by a U.S. F-15 fighter jet.

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The official was not authorized to be quoted by name and spoke on condition of anonymity. The official says the drone was a Shaheed 129 and appeared to have been operated by "pro-regime" forces.

It was the second time this month that the U.S. has shot down an armed drone in the vicinity of the camp at Tanf.