United States President Barack Obama says he has authorised air strikes against Islamic militants in northern Iraq, if they threaten US interests or to prevent the slaughter of religious minorities.

However, the president said US troops would be not be sent back to Iraq.

The US has already made humanitarian air drops to Iraqis under threat from Islamic State militants. IS has seized Qaraqosh, Iraq's biggest Christian town, prompting residents to flee, reports  BBC.

"We can act, carefully and responsibly, to prevent a potential act of genocide," Obama said, referring to the attacks against the besieged Yazidi minority, thousands of whom are trapped on a mountain in northern Iraq.

"I therefore authorised targeted air strikes if necessary to help forces in Iraq as they fight to break the siege and protect the civilians trapped there," Obama said.