Kim Jong Un was named vice chairman of the Central Military Commission and was appointed to the party's central committee, the BBC reported Wednesday.
Kim Jong Il, 68, believed to be in poor health, was re-elected as party leader during the Workers' Party's first convention in 20 years. Kim Jong Un is Kim Jong Il's third son and has been widely seen by observers as the most likely successor.
U.S. officials said the significance of the promotions weren't clear as yet.
"I would suppose this is perhaps the ultimate reality show unfolding in North Korea, and we are simply watching this very closely," State Department spokesman P.J. Crowley said.
The ruling party also introduced new politburo members as it wrapped up its convention Wednesday, South Korea's news agency Yonhap reported.
Notable among the new members was Ri Yong Ho, chief of the general staff of the North's Korean People's Army. Also named were Kim Yong Nam and Jo Myong Rok, both 82, joining the elder Kim.