Obama has demanded Muammar Gaddafi halt all military attacks against civilians, and says if the Libyan leader does not stand down the US will join in military action against him.
However, the President said he was “not going to deploy ground troops into Libya”.
In a brief appearance at the White House, Mr Obama said Secretary of State Hillary Rodham Clinton would travel to Paris to join in a meeting of allies called to discuss next steps in Libya, where Gaddafi has launched a brutal crackdown against rebels trying to end his 42-year reign.
Mr Obama’s remarks came less than 24 hours after the United Nations Security Council voted to authorise military action to prevent the killing of civilians by Libyan forces.
There should be no doubt about Gaddafi’s intentions “because he has made them clear”, Mr Obama said.
“Just yesterday, speaking of the city of Benghazi, a city of roughly 700,000, he threatened ‘we will have no mercy and no pity’. No mercy on his own citizens”.
The President has been criticised by US politicians and others for not moving more forcefully, while Gaddafi has regrouped in recent days and has taken the offensive against the rebels.
Mr Obama said the United States and other nations had imposed sanctions on Libya, frozen assets of the leader and delivered humanitarian supplies to bordering countries to help ease the plight of thousands fleeing the fighting.
“Now, once more, Muammar Gaddafi has a choice,” Mr Obama said, listing what he said were non-negotiable conditions laid out by the UN Security Council.
“If Gaddafi does not comply, the international community will impose consequences, and the resolution will be enforced through military action.” Agencies