See “Entry, Exit & Visa Requirements” on this US Department of State site
Comprehensive Information on Micronesia Visa Requirements
Comprehensive Information on Nauru Visa Requirements
Each island airport has its own customs and immigration officers. If you plan on visiting several islands on one trip, you will be passing through customs and immigration lines at each stop and filling out the usual paperwork. U.S. citizens may enter any FSM state with a U.S. passport...
Visitors to North Korea must obtain a visa from one of the North Korean diplomatic missions unless they come from one of the visa exempt countries.
North Korea entry and exit formalities – guide to North Korea visas, immigration, customs and border security
Comprehensive Information on Syria Visa Requirements
See “Entry, Exit & Visa Requirements on this US Department of State site
Comprehensive Information on Tuvalu Visa Requirements
Tuvalu visa and passport requirements & Embassy and Colsulate addresses. Full detail on passports, visas and entry requirements for Tuvalu, including types, costs