President Barack Obama said the sanctions were justified by Libya's "continued violation of human rights, brutalisation of its people and outrageous threats" that have drawn condemnation from the world.
"By any measure, Muammar Gaddafi's Government has violated international norms and common decency and must be held accountable," Obama said. He said the sanctions were designed to target Gaddafi's Government and protect the assets of Libya's people from being looted by the regime.
In an executive order detailing the sanctions and signed by Obama, the President said the instability in Libya constituted an "unusual and extraordinary threat" to US national security and foreign policy.
The sharper US tone and pledges of tough action came after American diplomatic personnel were evacuated from Tripoli aboard a chartered ferry and a chartered aircraft, escorting them away from the violence to Malta and Turkey.