US jets attacked jihadists who have besieged civilians on an Iraqi mountain for a week, as France and Britain Sunday joined a desperate race to save them from starvation.

Two days after Washington deployed its airforce over Iraq, a coordinated Western aid effort was shaping up to avert what US President Barack Obama warned could be an impending genocide.

An attack by extremist Islamic State (IS) militants on the Sinjar region a week ago sent thousands -- many of them from the Yazidi minority -- scurrying into a nearby mountain.

Most have since been stranded on Mount Sinjar in searing summer heat with little food and water. A Yazidi leader warned Saturday that they would not survive much longer.

US forces "successfully (conducted) four airstrikes to defend Yazidi civilians being indiscriminately attacked" near Sinjar, the US military said late Saturday.

Obama has said he was confident the US airforce could prevent IS fighters "from going up the mountain and slaughtering the people who are there" but added the next step of creating a safe passage was "logistically complicated".

US and Iraqi cargo planes have been air dropping food and water over Mount Sinjar, a barren 60-kilometre (35 miles) ridge that local legend holds as the final resting place of Noah's Ark.

French Foreign Minister Laurent Fabius was on his way to Iraq, where he is due to oversee the first delivery of French aid for displaced people in the Sinjar area.

- Air drops -

Britain also chipped in with two planes headed to Iraq with water, tents, tarpaulins and solar lights that can also recharge mobile phones.

"The United States can't just look away. That's not who we are. We're Americans. We act. We lead. And that's what we're going to do on that mountain," Obama said Saturday.

But many civilians have been cowering in caves and are scattered across the range.

Vian Dakhil, a Yazidi member of Iraq's parliament, warned Saturday that anything short of a mass evacuation was unlikely to save all those who are stranded on the mountain.

Several thousand of mainly Yazidi civilians have managed to flee the mountain but a majority, including the weakest among the displaced, remain trapped.

An alliance of Kurdish fighters from Iraq, Syria and Turkey has escorted Yazidis and members of other minorities to safety but IS remains largely in control of the area.

At pains to assure war-weary Americans he was not being dragged into a new Iraqi quagmire, Obama put the onus on Iraqi politicians to form an inclusive government and turn the tide on jihadist expansion which has brought Iraq closer to breakup than ever.

His comments were yet another nudge for Prime Minister Nuri al-Maliki to step aside and allow for a consensus government by abandoning what looks like an increasingly desperate bid to seek a third term.

- Call for reason -

UN Secretary General Ban Ki-moon also renewed a call "for reason and wisdom to prevail".

Obama said it would be "very hard to get a unified effort by Iraqis against" the jihadists without a broad-based government that can convince Sunni Iraqis that IS "is not the only game in town."

Federal Iraqi forces completely folded when IS militants who already control a large swathe of Syria swept in from the northeast two months ago, took the second city of Mosul and advanced into much of the country's Sunni heartland.

The cash-strapped autonomous Kurdish region's peshmerga force has also struggled and turning Sunni Arabs against the jihadists is seen as the key to rolling back two months of losses.

Obama did not give a timetable for the US military intervention but said Saturday Iraq's problems would not be solved in weeks. "This is going to be a long-term project," he said.

Kurdish and federal officials have welcomed the US strikes as a much-needed morale boost and an opportunity to regroup and plan a joint fightback.

Hundreds of thousands of civilians have fled their homes, including in an area between Mosul and Kurdistan where Iraq's largest Christian town was completely emptied of its population, in the past week alone.

Most of them fled to the autonomous Kurdish region, where most of the aid and military effort against IS is being coordinated.