No action needs to be taken by existing clients. The following text has been taken directly from the U.S. Department of the Treasury website for maximum accuracy. Further information can be found here.
"The Office of Foreign Assets Control (OFAC) of the US Department of the Treasury 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. OFAC acts under Presidential national emergency powers, as well as authority granted by specific legislation, to impose controls on transactions and freeze assets under US jurisdiction. Many of the sanctions are based on United Nations and other international mandates, are multilateral in scope, and involve close cooperation with allied governments."
"The Treasury Department has a long history of dealing with sanctions. Dating back prior to the War of 1812, Secretary of the Treasury Gallatin administered sanctions imposed against Great Britain for the harassment of American sailors. During the Civil War, Congress approved a law which prohibited transactions with the Confederacy, called for the forfeiture of goods involved in such transactions, and provided a licensing regime under rules and regulations administered by Treasury.
OFAC is the successor to the Office of Foreign Funds Control (the "FFC"), which was established at the advent of World War II following the German invasion of Norway in 1940. The FFC program was administered by the Secretary of the Treasury throughout the war. The FFC's initial purpose was to prevent Nazi use of the occupied countries' holdings of foreign exchange and securities and to prevent forced repatriation of funds belonging to nationals of those countries. These controls were later extended to protect assets of other invaded countries. After the United States formally entered World War II, the FFC played a leading role in economic warfare against the Axis powers by blocking enemy assets and prohibiting foreign trade and financial transactions.
OFAC itself was formally created in December 1950, following the entry of China into the Korean War, when President Truman declared a national emergency and blocked all Chinese and North Korean assets subject to U.S. jurisdiction."