President Barack Obama has warned that the US offensive in Iraq is a “long-term project” to rout out militants and deliver aid to beleaguered civilians.

Recognising there was no US military solution to reverse Islamic State fighters’ advances in Iraq, Obama on Saturday called on Iraqi officials to urgently form a unity government.

While US air strikes have destroyed the militants’ arms and equipment within striking distance of the autonomous region of Kurdistan, Obama said the operations that began this week could last “months.”

He spoke as Iraqi forces prepared a US-backed counter-offensive.

“We feel confident we can prevent ISIL from going up the mountain and slaughtering the people who are there,” Obama said, using the militant group’s former name of Islamic State of Iraq and the Levant.

“But the next step, which is going to be complicated logistically, is how do we give safe passage for people down from the mountain and where can we ultimately relocate them so that they are safe.”

In a significant boost to efforts to help the civilians stranded on Mount Sinjar, Obama said British Prime Minister David Cameron and French President Francois Hollande have agreed to lend their support following telephone talks.

Obama has justified the US intervention by warning of the risk of genocide against the small Yazidi minority, many of whose members have been trapped on the mountain for a week in northern Iraq.

But the president repeated assurances that no US combat troops would be deployed to Iraq in the first American offensive since Washington pulled out its forces in 2011.

“I’m not going to give a particular timetable, because as I’ve said from the start, wherever and whenever US personnel and facilities are threatened, it’s my obligation, my responsibility as commander in chief, to make sure they are protected,” Obama said.

Stressing that Iraq’s deep divisions along ethnic and religious lines were undermining the effort to combat IS, Obama said that it would be easier to mobilise Iraqis against the threat and obtain international support once an inclusive government is in place.

“Ultimately, only Iraqis can ensure the security and stability of Iraq, and the United States can’t do it for them, but we can and will be partners in that effort,” he said.