President Barack Obama said that Colonel Muammar Gaddafi's "rule is over" and that while some uncertainty remained it was clear that "the future of Libya is in the hands of its people".
In his first appearance since the rebel push towards Tripoli, Mr Obama interrupted his Martha's Vineyard holiday to appeal to Col Gaddafi give himself up and prevent further bloodshed in Libya.
He urged opposition forces to build a democratic government through "peaceful, inclusive and just" measures.
"For over four decades, the Libyan people had lived under the rule of a tyrant who denied them their most basic human rights," he said.
"Now the celebrations that we've seen in the streets of Libya shows that the pursuit of human dignity is far stronger than any dictator."
He stressed that "true justice will not come from reprisals and violence, it will come from reconciliation and a Libya that allows its citizens to determine their own destiny" while promising that the US would "be a friend and a partner" to a new Libyan regime.
"We do not have information that he's left the country," said Colonel Dave Lapan, a Pentagon spokesman.
The situation was "fluid" and American defence officials were closely monitoring events.
He ruled out the deployment of any US troops as part of a UN or Nato force.
"There will not be US boots on the ground," he said. "If there is going to be some kind of transitional mission that involves any kind of foreign troops, there wouldn't be US ground troops."
The apparent collapse of the Gaddafi regime came at an awkward moment for Mr Obama, who has been lambasted for taking a break in Martha's Vineyard at a time of economic crisis.
The White House had stressed that he would be spending a lot of his time working on a speech about jobs scheduled for the start of September.More