US military aircraft conducted an airstrike on Friday against Islamic State artillery used against Kurdish forces defending the city of Erbil, near US personnel, a Pentagon spokesman said.

Two F/A-18 aircraft dropped 500-pound laser-guided bombs on a mobile artillery piece near Erbil, Pentagon press secretary Rear Admiral John Kirby said in a statement.

He said the Islamic rebels had been using the artillery to shell Kurdish forces defending Erbil where US personnel are located.

"The decision to strike was made by the US Central Command commander under authorization granted him by the commander in chief," he said.

On Thursday US President Barack Obama authorized air strikes on Iraq to protect Christians and prevent "genocide" of tens of thousands of members of an ancient sect sheltering on a desert mountaintop from Islamic State fighters threatening to exterminate them.

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