US aircraft have struck Iraq for the first time since American troops pulled out in 2011, attacking Islamist fighters advancing towards the Kurdish region after President Barack Obama said Washington must act to prevent "genocide".

Pentagon press secretary Rear Admiral John Kirby said: "The decision to strike was made by the US Central Command commander under authorisation granted him by the commander in chief."

The Islaminists had advanced to Arbil, capital of Iraq's Kurdish region and a hub for US oil companies.

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The Pentagon said two F/A-18 aircraft dropped bombs on a mobile artillery piece used by Islamic State, an al Qaeda offshoot determined on establishing a caliphate and eradicating non-believers.

Mr Obama authorised the action after tens of thousands of Christians fled for from Islamic State fighters who have crucified and beheaded captives.

"We can act carefully and responsibly to prevent a potential act of genocide," he said.

The United States also started to drop relief supplies to the Yazidi sect massed on a desert mountaintop seeking shelter from the fighters who had ordered them to convert or die.

Rear Admiral Kirby said the Islamic rebels had been shelling forces defending Arbil, the capital of Iraq's semi-autonomous Kurdish region.

The United States has a consulate and, since Iraq's latest security crisis erupted in June, a joint military operations centre there.

The strike was launched from the aircraft carrier USS George HW Bush. In June, the Pentagon ordered the ship to the Gulf in preparation for any possible military action in Iraq.

The strike came yesterday only hours after Mr Obama authorised airstrikes on Iraq late on Thursday.

The Sunni fighters from the Islamic State have swept through northern Iraq since June. They are now encroaching on Arbil. It is major city, seat of the Kurdish region's government, its parliament and, now, temporary home to scores of refugees who have fled other parts of Iraq.

The renewed military action in Iraq is an enormous turn-around two-and-a-half years after Mr Obama pulled all US troops out of Iraq in December 2011, ending the US war that began in 2003.

While Mr Obama has insisted the United States would not commit its ground troops again, since June he has ordered some 700 soldiers into Iraq to protect diplomatic personnel and facilities and to assess the strengths and weaknesses of Iraq's military, much of which retreated in the face of the Islamic State advance.

Also in June, the United States relocated some of its staff from the US Embassy in Baghdad to Arbil, Basra, and in Jordan, Amman, due to fears the Islamic State forces could directly attack the Iraqi capital.

In Baghdad, where politicians have been paralysed by infighting while the state falls apart, the top Shi'ite cleric all but demanded Prime Minister Nouri al Maliki quit, a bold intervention that could bring the veteran ruler down.

"Earlier this week, one Iraqi in the area cried to the world, 'There is no-one coming to help'," said Mr Obama in a TV broadcast. "Well, today America is coming to help."