WASHINGTON (AFP) - President Barack Obama said on Thursday he had authorised US air strikes on Iraq and humanitarian supply drops to prevent a "genocide" by Islamist extremists against minorities.

"We can act, carefully and responsibly, to prevent a potential act of genocide," Obama said, referring to the attacks against the besieged Yazidi minority.

"I therefore authorized 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.

Obama, who did not say whether air strikes have been carried out, said US forces have already started to drop food and water to Iraqis racing to flee the so-called Islamic State fighters.

"Earlier this week, one Iraqi in the area cried to the world, there is no one coming to help. Well, today America is coming to help," Obama said.

"When we face a situation like we do on that mountain, with innocent people facing the prospect of violence on a horrific scale, when we have a mandate to help - in this case a request from the Iraqi government - and when we have the unique capabilities to help avert a massacre, then I believe the United States of America cannot turn a blind eye," he said.

Obama, who rose to political prominence as an outspoken critic of his predecessor George W. Bush's 2003 invasion of Iraq, said he was not sending back ground forces.

"As commander in chief, I will not allow the United States to be dragged into fighting another war in Iraq.

"And so even as we support Iraqis as they take the fight to these terrorists, American combat troops will not be returning to fight in Iraq, because there is no American military solution to the larger crisis in Iraq," he said.