The Obama administration is freezing assets held by Libyan leader Moammar Gadhafi and four of his children in the United States.
The Treasury Department says the sanctions against Gadhafi, three of his sons and a daughter also apply to the Libyan government.
The action was taken under an executive order signed Friday by President Barack Obama.
Obama says the U.S. is imposing unilateral sanctions on Libya because continued violence there poses an "unusual and extraordinary threat" to America's national security and foreign policy.
The president acted after hundreds of Americans were safely evacuated from Libya following days of bloodshed across the country. Militias loyal to Gadhafi have been firing on protesters who have been demanding the Libyan leader's ouster.