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

Two F/A-18 aircraft dropped 500-pound laser-guided bombs on a mobile artillery piece near Arbil, 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 Arbil 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.

Kirby said the strike occurred at 6:45 am EDT, or 1:45 pm in Arbil (1045 GMT).

On Thursday US President Barack Obama authorized air strikes on Iraq to protect Christians and avert "a potential act of genocide" of tens of thousands of members of the ancient Yazidi sect who have taken refuge on a desert mountaintop from Islamic State forces.

The United States has also begun dropping relief supplies to the refugees.

Sunni fighters from the Islamic State, an al-Qaida offshoot bent on establishing a caliphate and eradicating unbelievers, have swept through northern Iraq since June.

US troops pulled out of Iraq in 2011 after having ousted ruler Saddam Hussein eight years earlier. Obama insisted the United States would not commit ground troops.