President Barack Obama said the US is imposing unilateral sanctions on Libya because continued violence and unrest there poses an "unusual and extraordinary threat" to America's national security and foreign policy.
He said a series of financial sanctions he formalised target the government of Libyan leader Muammar Gaddafi while protecting the Libyan people's assets.
The penalties name Mr Gaddafi and several family members.
Mr Obama acted after hundreds of Americans were safely evacuated from Libya following days of bloodshed across the country.
Militias loyal to Mr Gaddafi have been firing on protesters who have been demanding the Libyan leader's ousting.
White House spokesman Jay Carney said earlier that the US would impose unilateral sanctions and continue to work with its European partners on additional punishments that could be taken.
Earlier, a US official said the United States has closed its embassy in Libya's capital after evacuating all personnel on a ferry and a chartered plane.
The Tripoli embassy's operations were suspended because of the deteriorating security situation, the official said.
The last embassy staff departed on a chartered flight at 6.49pm (UK time) on Friday.