It's come up before but I believe that there is a statement on the website which prohibits us from booking to certain countries on the US's bad list, even if flying on other carriers, using UA miles.
http://www.flyertalk.com/forum/unite...estions-4.html
Edited to add that reading the linked thread, I found my own post but it was in reference to GPU on LH
UA does however prohibit using GPUs on LH flights to Cuba
Quote:
*As of March 25, 2013, U.S. law prohibits/restricts travel to destinations within the following countries: Afghanistan, Cuba, Iran, North Korea, Syria and Yemen. Global Premier Upgrades will not be issued for use on Lufthansa-operated flights to destinations within these countries.