An office of the US Department of the Treasury.

http://www.treasury.gov/about/organizational-structure/offices/Pages/Office-of-Foreign-Assets-Control.aspx

The Office of Foreign Assets Control (OFAC) is responsible for enforcing economic and trade sanctions against targeted foreign states, organizations and individuals.

OFAC administers and enforces economic and trade sanctions based on US foreign policy and national security goals against targeted foreign countries and regimes, terrorists, international narcotics traffickers, those engaged in activities related to the proliferation of weapons of mass destruction, and other threats to the national security, foreign policy or economy of the United States.

FAQ List of Sanctions Programs

All US persons must comply with OFAC regulations, including all US citizens and permanent resident aliens regardless of where they are located, all persons and entities within the United States, all US incorporated entities and their foreign branches.

http://en.wikipedia.org/wiki/Office_of_Foreign_Assets_Control