He also announced the use of airstrikes to protect humanitarian aid to mainly religious minorities who had moved to and become trapped in mountaintops.
"We do whatever is necessary to protect our people. We support our allies when they're in danger," Obama said.
"When we have the unique capabilities to help avert a massacre, then I believe the United States of America cannot turn a blind eye," he added.
Kurdish and Iraqi officials said the airstrikes had already taken place, though the Pentagon denied any attacks had started, The New York Times reported.
The announcement comes as religious minorities in Iraq, including Christians, fled Sunni Muslim militants operating under the name the Islamic State, formerly known as the Islamic State of Iraq and Syria, or ISIS.