"We feel confident" that military efforts can prevent ISIS from slaughtering people on Mount Sinjar, where ISIS has been killing many members of the Yazidi minority, Obama said in remarks at the White House.
He declined to give a timetable for U.S. airstrikes and humanitarian air drops in Iraq. "Wherever and whenever (U.S.) personnel are facilities are threatened, it is my obligation ... to make sure they're protected," he said.
The Iraqi government and military will need to take a series of steps to improve the security situation, Obama said. "I don't think we're going to solve this problem in weeks. I think this is going to take some time."
There's a lesson in this situation for Afghanistan, the president said: If leadership wants a new government to work, then people of different factions and ethnicities have "got to accommodate each other."
After a reporter asked Obama if he felt ISIS had been underestimated, Obama said the advance of the Sunni Islamic extremists has been "more rapid" than intelligence officials and policymakers in and outside Iraq had predicted.
But he said ISIS' advance was made possible in part by the lack of an inclusive and functioning Iraqi government -- a dig at the Shiite-led government that he says has alienated Sunnis in recent years.
This was illustrated, he said, by Iraqi security forces' capitulation to ISIS in northern Iraq earlier this year. The government forces, "when they (were) far away from Baghdad, did not have the incentive or the capacity to ... hold (their) ground against an aggressive adversary," Obama said.
"That's one more reason why" the formation of a functioning Iraqi government is so important, he said.
While the U.S. can assist Iraqi security forces in fighting ISIS with airstrikes, ultimately it's up to Iraqis to secure their country, the president said. That will take an inclusive government, he said. "All Iraqi communities need to unite to defend their country," Obama said.
Obama said he spoke with French President Francois Hollande and British Prime Minister David Cameron about the situation in Iraq, and that both have agreed to join the United States in providing humanitarian assistance to Iraqis endangered by ISIS.
The British government will contribute aid worth 8 million pounds ($13.4 million), and its air force will start air-dropping supplies in northern Iraq soon, particularly for the Yazidis trapped on the mountain, UK Foreign Secretary Philip Hammond said Saturday.