Upvote:6
Unless your last entry to Kosovo was from Serbia (Serbia-Kosovo-Serbia), you'll be refused entry to Serbia from Kosovo on a foreign passport as the Serbs don't issue entry stamps at those checkpoints, with a Kosovan entry stamp NOT deemed a valid substitute. An ID card would've done the trick for eligible nationalities, but US passport cards aren't valid for Serbia.
So you indeed have to travel via Macedonia (or Montenegro)
Upvote:16
Please note: when you are in Kosovo, you already entered Serbia, from the serbian legal perspective.
If you directly enter Kosovo from outside Serbia, from the Serbian perspective you never officially and legally entered Serbia. Therefore, if you manage or try to enter Serbia from Kosovo, it means (from the serbian perspective) you already entered illegally in Serbia. So they will not allow you to stay in Serbia (from their point of view Kosovo is Serbia). Even less to further travel through Serbia.
See the informations provided by the serbian consulate in Lebanon: http://www.beirut.mfa.gov.rs/consularservicestext.php?subaction=showfull&id=1581599779&ucat=17&template=MeniENG&