American aircraft bombed positions held by Islamic State insurgents in northern Iraq on Friday in the first major US military action since it withdrew troops, the Pentagon said.

A day after President Barack Obama publicly authorized the use of force to avert a "genocide," US jets struck after guerrillas shelled positions near the Kurdish region's capital of Arbil.

Two US F/A-18 aircraft dropped 500-pound (225-kilogram) laser-guided bombs on a mobile artillery piece of Islamic State of Iraq and the Levant, the Sunni extremist movement that has swept across Iraq and Syria, Pentagon spokesman Rear Admiral John Kirby said.

Kirby said that the strike, carried out at 1045 GMT, was ordered after the IS shelling was deemed a risk to US personnel based in Arbil, long considered a safe city in the troubled country.

"As the president made clear, the United States military will continue to take direct action against ISIL when they threaten our personnel and facilities," Kirby said in a statement.

In Iraq, army chief of staff Lieutenant General Babaker Zebari hailed the strikes, telling AFP that the raid will mean "huge changes on the ground."

Obama pledged a limited mission to defend Arbil and break the IS siege of a group of thousands of members of the Yazidi minority fleeing the combat and huddled exposed on a mountain side.

On Thursday, the United States dropped thousands of gallons of drinking water and 8,000 packaged meals to Yazidis who risk starvation as they cram onto Mount Sinjar.

"We can act, carefully and responsibly, to prevent a potential act of genocide," Obama said in a televised address on Thursday night.

"We plan to stand vigilant and take action if they threaten our facilities anywhere in Iraq, including the consulate in Arbil and embassy in Baghdad," he said.

But Obama, who rose to political prominence as an outspoken critic of his predecessor George W. Bush's 2003 invasion of Iraq, said he would not send US ground forces back into the country.

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

"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.

And US officials have made clear they do not anticipate a long-term campaign to eliminate IS as they press Iraq's Shiite-dominated government to reach out to the Sunni minority.

"We are not launching a sustained US campaign against ISIL here, because our belief is the best way to deal with the threat of ISIL over the long term is for the Iraqis to do so," a US official said Thursday on condition of anonymity.

The official said that Obama had given orders to strike if the so-called Islamic State threatens Arbil or US facilities.

He also said that the United States was also ready to strike if Iraqi government troops and the Kurdish forces known as the peshmerga fail to break a siege at a mountain where thousands of Yazidi civilians have huddled.

"The Iraqi security forces and the peshmerga are working to break that siege, but, as with the safety of Arbil, if we see a need to take direct US military action through air strikes to relieve the pressure on the Yazidis," the official said.