The US military has conducted two additional air strikes in Iraq on Islamic State targets, according to the Pentagon.

It said the air strikes included a drone strike on a mortar position and a strike by fighter jets on an Islamic State convoy near Arbil.

Today is the first day US warplanes have bombed Islamist fighters marching on Iraq's Kurdish capital after President Barack Obama said Washington must act to prevent "genocide".

Islamic State fighters have advanced to within a half hour's drive of Arbil, capital of Iraq's Kurdish region and a hub for US oil companies.

They have also seized control of Iraq's biggest dam, which could allow them to flood cities and cut off vital water and electricity supplies.

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

Mr Obama this week authorised the first US air strikes on Iraq since he pulled all troops out in 2011.

He argued 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 US 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 Mr Obama said last night. "Well, today America is coming to help."

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

Today, the White House said the strikes would last as long as the security situation required.

The Islamic State was defiant. A fighter told Reuters by telephone that the US air strikes would have "no impact on us".

"The planes attack positions they think are strategic, but this is not how we operate. We are trained for guerrilla street war," he said.

"God is with us and our promise is heaven. When we are promised heaven, do you think death will stop us?"            

The advance of the Sunni militants has sounded alarm across the Middle East and threatens to unravel Iraq, a country divided between Shias, Sunnis and Kurds.

The US air strikes prompted renewed calls on jihadi online forums for attacks on the United States and oil interests in the Gulf.

Politicians in Baghdad have been paralysed by infighting while the state falls apart.

The top Shia cleric there has now all but demanded that Iraqi Prime Minister Nouri al-Maliki quit, a bold intervention that could bring the veteran ruler down.

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.

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.

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.