The Swiss-educated Kim Jong-un was promoted ahead of the ruling party's biggest meeting in decades.
It has fuelled rumours that the 27-year-old — whose name has never been mentioned in official state media until now — will be anointed successor to his communist dictator father.
Kim Jong-Il, 68, is reportedly suffering from several illnesses, but Pyongyang never comments publicly on his health.
The succession is being closely watched by the West due to North Korea's weapons program.
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