The Treasury Department's Office of Foreign Assets Control (OFAC) administers and enforces economic sanctions imposed by the United States against foreign countries. Depending on the country, OFAC programs may freeze assets of embargoed countries, prohibit payment of funds to individuals and countries on the embargo list, or prohibit provision of services to countries subject to US sanctions. These sanctions may require obtaining OFAC approval before conducting research or other activities in or involving the sanctioned country. Some sanctions are more restrictive than others, and apply to the whole country, while others are specifically target certain individuals or entities within a country. The list of sanctioned countries is updated periodically and is available here.
OFAC can also designate persons and entities (including persons and entities in the United States) as Specially Designated Nationals ("SDN list"). OFAC designates persons and entities as SDNs for narcotics trafficking, weapons proliferation and other reasons. When entering into discussions with a proposed collaborator, it is critical to check the SDN list for the name of the person or entity with which you are dealing.