The Maritime Transportation Security Act (MTSA) authorizes the Coast Guard (USCG) to impose conditions of entry on vessels arriving in U.S. waters from ports that the Coast Guard has not found to maintain effective anti-terrorism measures. 
The USCG publishes a list of countries found not to meet minimum security standards in a Port Security Advisory (PSA).
The PSA provides Conditions of Entry which are measures any vessel who visits one of the listed countries (in last 5 ports) must take before entering US Ports.
UpdateThe Coast Guard announces (see herethat it is removing the conditions of entry on vessels arriving from the country of the Republic of Liberia.
With this notice, the current list of countries assessed and not maintaining effective anti-terrorism measures is as follows: 
  • Cambodia,
  • Cameroon, 
  • Comoros, 
  • Cote d’Ivoire,
  • Djibouti, 
  • Equatorial Guinea, 
  • The Gambia, 
  • Guinea-Bissau, 
  • Iran, 
  • Iraq,
  • Libya, 
  • Madagascar, 
  • Micronesia, 
  • Nauru, 
  • Nigeria, 
  • Republic of Seychelles, 
  • Sao Tome and Principe, 
  • Syria, 
  • Timor-Leste,
  • Venezuela, 
  • Yemen. 
The current Port Security Advisory is available here.