US warplanes bombed Islamist fighters after President Barack Obama call to prevent genocide

US warplanes bombed Islamist fighters marching on Iraq's Kurdish capital on Friday after President Barack Obama said Washington must act to prevent "genocide".

Islamic State fighters, who have beheaded and crucified captives in their drive to eradicate unbelievers, have advanced to within a half hour's drive of Arbil, capital of Iraq's Kurdish region and a hub for US oil companies.

A Pentagon spokesman said two F/A-18 aircraft from an aircraft carrier in the Gulf had dropped laser-guided 500-pound bombs on a mobile artillery piece used by the fighters to shell Kurdish forces defending Arbil.

Obama authorised the first US air strikes on Iraq since he pulled all troops out in 2011, arguing action was needed to halt the Islamist advance, protect Americans and safeguard hundreds of thousands of Christians and members of other religious minorities who have fled for their lives.

The United States also dropped relief supplies to members of the ancient Yazidi sect, tens of thousands of whom are massed on a desert mountaintop seeking shelter from fighters who had ordered them to convert or die.

"Earlier this week, one Iraqi in the area cried to the world, 'There is no one coming to help'," said Obama in a late night television address to the nation on Thursday. "Well, today America is coming to help."

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

The advance of the Sunni militants, who also control a third of Syria and have fought this past week in Lebanon, has sounded alarm across the Middle East and threatens to unravel Iraq, a country divided between Shi'ites, Sunnis and Kurds.

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

SHELTERING ON MOUNTAIN

Sunni fighters from the Islamic State, an al Qaeda offshoot rejected as too extreme by Osama bin Laden's successors, have swept through northern Iraq since June. Their advance has dramatically accelerated in the past week when they routed Kurdish troops near the Kurdish autonomous region in the north.

Attention has focused on the plight of Yazidis, Christians and other minority groups in northern Iraq, which has been one of the most diverse parts of the Middle East for centuries.

"The stakes for Iraq's future can also not be clearer," US Secretary of State John Kerry said on Friday. The Islamic State's "campaign of terror against the innocent, including the Christian minority, and its grotesque targeted acts of violence show all the warning signs of genocide."

The US Defense Department said planes dropped 72 bundles of supplies, including 8,000 ready-to-eat meals and thousands of gallons of drinking water, for threatened civilians near Sinjar, home of the Yazidis, ethnic Kurds who practice an ancient faith related to Zoroastrianism.

The Islamic State considers them to be "devil worshippers". After fighters ordered them to leave, convert or die, most fled their towns and villages to camp out on Sinjar mountain, an arid peak where they believe Noah settled after the biblical flood.

"After we fled to the mountain, I returned one day to recover belongings and I saw the bodies of the elderly disabled men who had been shot dead by the Islamic State. They were too old to flee. I can't forget that scene," said Akram Edo, who escaped to Kurdish-held territory with seven children.

His brother Hameed Edo, still back on the mountain with five children, told Reuters by telephone water was running out and no aid had arrived for the civilians trapped in the wilderness.

Mahma Khalil, a Yazidi lawmaker in Baghdad, said: "We hear through the media there is American help, but there is nothing on the ground. Please save us! SOS! save us!" he said. "Our people are in the desert. They are exposed to a genocide."