Office of Foreign Assets Control (OFAC)

U.S. Department of the Treasury, Office of Foreign Assets Control (OFAC) 
U.S. Sanctions and Specially Designated Nationals List 
31 CFR 500-599

The U.S. Department of the Treasury, OFAC, administers economic and trade sanctions against targeted foreign countries and regimes, terrorists, narcotics traffickers, and other threats to the national security, foreign policy or economy of the United States. OFAC has a broad authority to block prohibited transactions involving restricted countries or parties. Many of the sanctions are based on United Nations or other international mandates and are performed in cooperation with allied governments. 

Normally OFAC supersedes export controls. OFAC prohibits travel to and other activities with embargoes countries and entities.