A former US Marine detained in Russia is pleading to come home, after being left out of a prisoner swap that freed star American basketballer Brittney Griner.

Russia agreed to release the Olympic gold medallist in exchange for its notorious arms dealer Viktor Bout.

But American Paul Whelan wants to know why after nearly four years in jail, his freedom wasn't part of the package. 

This report by ABC's PM includes comments by USSC chief executive Michael Green.