Recognising there was no US military solution to reverse Islamic State fighters' advances in Iraq, Obama 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.
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In a significant boost to efforts to help the civilians stranded on Mount Sinjar, the 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.
"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 told reporters.
The Islamic State's march on Kurdistan is placing US diplomats -- including at the American consulate -- and business people at risk in a region that was relatively safe from the violence.